diff --git a/library/algebra/ordered_field.lean b/library/algebra/ordered_field.lean
index 4a6672a9e..70b304d9a 100644
--- a/library/algebra/ordered_field.lean
+++ b/library/algebra/ordered_field.lean
@@ -329,6 +329,8 @@ section discrete_linear_ordered_field
       (assume H' : ¬ y < x,
         decidable.inl (le.antisymm (le_of_not_gt H') (le_of_not_gt H))))
 
+  reveal dec_eq_of_dec_lt
+
   definition discrete_linear_ordered_field.to_discrete_field [trans-instance] [reducible] [coercion]
      :  discrete_field A :=
      ⦃ discrete_field, s, has_decidable_eq := dec_eq_of_dec_lt⦄
diff --git a/library/data/empty.lean b/library/data/empty.lean
index 1cda7d48d..27ecd4e54 100644
--- a/library/data/empty.lean
+++ b/library/data/empty.lean
@@ -13,6 +13,8 @@ namespace empty
   subsingleton.intro (λ a b, !empty.elim a)
 end empty
 
+reveal empty.elim
+
 protected definition empty.has_decidable_eq [instance] : decidable_eq empty :=
 take (a b : empty), decidable.inl (!empty.elim a)
 
diff --git a/library/data/quotient/classical.lean b/library/data/quotient/classical.lean
index bbf89c2cf..ceb847574 100644
--- a/library/data/quotient/classical.lean
+++ b/library/data/quotient/classical.lean
@@ -14,7 +14,7 @@ open relation nonempty subtype
 
 /- abstract quotient -/
 
-definition prelim_map {A : Type} (R : A → A → Prop) (a : A) :=
+noncomputable definition prelim_map {A : Type} (R : A → A → Prop) (a : A) :=
 -- TODO: it is interesting how the elaborator fails here
 -- epsilon (fun b, R a b)
 @epsilon _ (nonempty.intro a) (fun b, R a b)
@@ -42,12 +42,12 @@ assert H5 : nonempty A, from
 show epsilon (λc, R a c) = epsilon (λc, R b c), from
   congr_arg _ H4
 
-definition quotient {A : Type} (R : A → A → Prop) : Type := image (prelim_map R)
+noncomputable definition quotient {A : Type} (R : A → A → Prop) : Type := image (prelim_map R)
 
-definition quotient_abs {A : Type} (R : A → A → Prop) : A → quotient R :=
+noncomputable definition quotient_abs {A : Type} (R : A → A → Prop) : A → quotient R :=
 fun_image (prelim_map R)
 
-definition quotient_elt_of {A : Type} (R : A → A → Prop) : quotient R → A := elt_of
+noncomputable definition quotient_elt_of {A : Type} (R : A → A → Prop) : quotient R → A := elt_of
 
 theorem quotient_is_quotient  {A : Type} (R : A → A → Prop) (H : is_equivalence R)
   : is_quotient R (quotient_abs R) (quotient_elt_of R) :=
diff --git a/library/data/real/complete.lean b/library/data/real/complete.lean
index 4df79a9ea..2bff36f60 100644
--- a/library/data/real/complete.lean
+++ b/library/data/real/complete.lean
@@ -216,9 +216,9 @@ theorem rewrite_helper9 (a b c : ℝ) : b - c = (b - a) - (c - a) :=
 theorem rewrite_helper10 (a b c d : ℝ) : c - d = (c - a) + (a - b) + (b - d) :=
   by rewrite[*add_sub,*sub_add_cancel]
 
-definition rep (x : ℝ) : s.reg_seq := some (quot.exists_rep x)
+noncomputable definition rep (x : ℝ) : s.reg_seq := some (quot.exists_rep x)
 
-definition re_abs (x : ℝ) : ℝ :=
+noncomputable definition re_abs (x : ℝ) : ℝ :=
   quot.lift_on x (λ a, quot.mk (s.r_abs a)) (take a b Hab, quot.sound (s.r_abs_well_defined Hab))
 
 theorem r_abs_nonneg {x : ℝ} : zero ≤ x → re_abs x = x :=
@@ -251,7 +251,7 @@ theorem rat_approx' (x : ℝ) : ∀ n : ℕ+, ∃ q : ℚ, re_abs (x - const q)
 theorem rat_approx (x : ℝ) : ∀ n : ℕ+, ∃ q : ℚ, abs (x - const q) ≤ const n⁻¹ :=
   by rewrite -re_abs_is_abs; apply rat_approx'
 
-definition approx (x : ℝ) (n : ℕ+) := some (rat_approx x n)
+noncomputable definition approx (x : ℝ) (n : ℕ+) := some (rat_approx x n)
 
 theorem approx_spec (x : ℝ) (n : ℕ+) : abs (x - (const (approx x n))) ≤ const n⁻¹ :=
   some_spec (rat_approx x n)
@@ -261,10 +261,10 @@ theorem approx_spec' (x : ℝ) (n : ℕ+) : abs ((const (approx x n)) - x) ≤ c
 
 notation `r_seq` := ℕ+ → ℝ
 
-definition converges_to (X : r_seq) (a : ℝ) (N : ℕ+ → ℕ+) :=
+noncomputable definition converges_to (X : r_seq) (a : ℝ) (N : ℕ+ → ℕ+) :=
   ∀ k : ℕ+, ∀ n : ℕ+, n ≥ N k → abs (X n - a) ≤ const k⁻¹
 
-definition cauchy (X : r_seq) (M : ℕ+ → ℕ+) :=
+noncomputable definition cauchy (X : r_seq) (M : ℕ+ → ℕ+) :=
   ∀ k : ℕ+, ∀ m n : ℕ+, m ≥ M k → n ≥ M k → abs (X m - X n) ≤ const k⁻¹
 --set_option pp.implicit true
 --set_option pp.coercions true
@@ -312,7 +312,7 @@ theorem Nb_spec_right (M : ℕ+ → ℕ+) (k : ℕ+) : M (2 * k) ≤ Nb M k := !
 
 theorem Nb_spec_left (M : ℕ+ → ℕ+) (k : ℕ+) : 3 * k ≤ Nb M k := !max_left
 
-definition lim_seq {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : ℕ+ → ℚ :=
+noncomputable definition lim_seq {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : ℕ+ → ℚ :=
   λ k, approx (X (Nb M k)) (2 * k)
 
 theorem lim_seq_reg_helper {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) {m n : ℕ+}
@@ -368,14 +368,14 @@ theorem lim_seq_spec {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) (k : ℕ+
     apply lim_seq_reg
   end
 
-definition r_lim_seq {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : s.reg_seq :=
+noncomputable definition r_lim_seq {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : s.reg_seq :=
   s.reg_seq.mk (lim_seq Hc) (lim_seq_reg Hc)
 
 theorem r_lim_seq_spec {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) (k : ℕ+) :
         s.r_le (s.r_abs (( s.radd (r_lim_seq Hc) (s.rneg (s.r_const ((s.reg_seq.sq (r_lim_seq Hc)) k)))))) (s.r_const (k)⁻¹) :=
   lim_seq_spec Hc k
 
-definition lim {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : ℝ :=
+noncomputable definition lim {X : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy X M) : ℝ :=
   quot.mk (r_lim_seq Hc)
 
 theorem re_lim_spec {x : r_seq} {M : ℕ+ → ℕ+} (Hc : cauchy x M) (k : ℕ+) :
diff --git a/library/data/real/division.lean b/library/data/real/division.lean
index f8afcdb4f..bcb3d08cd 100644
--- a/library/data/real/division.lean
+++ b/library/data/real/division.lean
@@ -107,9 +107,9 @@ theorem sep_zero_of_pos {s : seq} (Hs : regular s) (Hpos : pos s) : sep s zero :
 ------------------------
 -- This section could be cleaned up.
 
-definition pb {s : seq} (Hs : regular s) (Hpos : pos s) :=
+noncomputable definition pb {s : seq} (Hs : regular s) (Hpos : pos s) :=
   some (abs_pos_of_nonzero Hs (sep_zero_of_pos Hs Hpos))
-definition ps {s : seq} (Hs : regular s) (Hsep : sep s zero) :=
+noncomputable definition ps {s : seq} (Hs : regular s) (Hsep : sep s zero) :=
   some (abs_pos_of_nonzero Hs Hsep)
 
 
@@ -121,7 +121,7 @@ theorem ps_spec {s : seq} (Hs : regular s) (Hsep : sep s zero) :
         ∀ m : ℕ+, m ≥ (ps Hs Hsep) → abs (s m) ≥ (ps Hs Hsep)⁻¹ :=
   some_spec (abs_pos_of_nonzero Hs Hsep)
 
-definition s_inv {s : seq} (Hs : regular s) (n : ℕ+) : ℚ :=
+noncomputable definition s_inv {s : seq} (Hs : regular s) (n : ℕ+) : ℚ :=
   if H : sep s zero then
       (if n < (ps Hs H) then 1 / (s ((ps Hs H) * (ps Hs H) * (ps Hs H)))
         else 1 / (s ((ps Hs H) * (ps Hs H) * n)))
@@ -570,7 +570,7 @@ theorem s_le_of_equiv_le_right {s t u : seq} (Hs : regular s) (Ht : regular t) (
 
 -----------------------------
 
-definition r_inv (s : reg_seq) : reg_seq := reg_seq.mk (s_inv (reg_seq.is_reg s))
+noncomputable definition r_inv (s : reg_seq) : reg_seq := reg_seq.mk (s_inv (reg_seq.is_reg s))
   (if H : sep (reg_seq.sq s) zero then reg_inv_reg (reg_seq.is_reg s) H else
     have Hz : s_inv (reg_seq.is_reg s) = zero, from funext (λ n, dif_neg H), Hz⁻¹ ▸ zero_is_reg)
 
@@ -605,7 +605,7 @@ end s
 namespace real
 open [classes] s
 
-definition inv (x : ℝ) : ℝ := quot.lift_on x (λ a, quot.mk (s.r_inv a))
+noncomputable definition inv (x : ℝ) : ℝ := quot.lift_on x (λ a, quot.mk (s.r_inv a))
            (λ a b H, quot.sound (s.r_inv_well_defined H))
 postfix [priority real.prio] `⁻¹` := inv
 
@@ -651,7 +651,7 @@ theorem dec_lt : decidable_rel lt :=
 section migrate_algebra
   open [classes] algebra
 
-  protected definition discrete_linear_ordered_field [reducible] :
+  protected noncomputable definition discrete_linear_ordered_field [reducible] :
       algebra.discrete_linear_ordered_field ℝ :=
   ⦃ algebra.discrete_linear_ordered_field, real.comm_ring, real.ordered_ring,
     le_total := le_total,
@@ -667,11 +667,11 @@ section migrate_algebra
   local attribute real.comm_ring [instance]
   local attribute real.ordered_ring [instance]
 
-  definition abs (n : ℝ) : ℝ := algebra.abs n
-  definition sign (n : ℝ) : ℝ := algebra.sign n
-  definition max (a b : ℝ) : ℝ   := algebra.max a b
-  definition min (a b : ℝ) : ℝ   := algebra.min a b
-  definition divide (a b : ℝ): ℝ := algebra.divide a b
+  noncomputable definition abs (n : ℝ) : ℝ := algebra.abs n
+  noncomputable definition sign (n : ℝ) : ℝ := algebra.sign n
+  noncomputable definition max (a b : ℝ) : ℝ   := algebra.max a b
+  noncomputable definition min (a b : ℝ) : ℝ   := algebra.min a b
+  noncomputable definition divide (a b : ℝ): ℝ := algebra.divide a b
 
   migrate from algebra with real
     replacing has_le.ge → ge, has_lt.gt → gt, sub → sub, abs → abs, sign → sign, dvd → dvd,
diff --git a/library/data/set/classical_inverse.lean b/library/data/set/classical_inverse.lean
index 37f350a46..0e637d2a3 100644
--- a/library/data/set/classical_inverse.lean
+++ b/library/data/set/classical_inverse.lean
@@ -13,7 +13,7 @@ namespace set
 
 variables {X Y : Type}
 
-definition inv_fun (f : X → Y) (a : set X) (dflt : X) (y : Y) : X :=
+noncomputable definition inv_fun (f : X → Y) (a : set X) (dflt : X) (y : Y) : X :=
 if H : ∃₀ x ∈ a, f x = y then some H else dflt
 
 theorem inv_fun_pos {f : X → Y} {a : set X} {dflt : X} {y : Y}
@@ -65,7 +65,7 @@ namespace map
 
 variables {X Y : Type} {a : set X} {b : set Y}
 
-protected definition inverse (f : map a b) {dflt : X} (dflta : dflt ∈ a) :=
+protected noncomputable definition inverse (f : map a b) {dflt : X} (dflta : dflt ∈ a) :=
 map.mk (inv_fun f a dflt) (@maps_to_inv_fun _ _ _ _ b _ dflta)
 
 theorem left_inverse_inverse {f : map a b} {dflt : X} (dflta : dflt ∈ a) (H : map.injective f) :
diff --git a/library/init/logic.lean b/library/init/logic.lean
index 9cef322f9..ed198ac1b 100644
--- a/library/init/logic.lean
+++ b/library/init/logic.lean
@@ -18,7 +18,7 @@ definition trivial := true.intro
 definition not (a : Prop) := a → false
 prefix `¬` := not
 
-theorem absurd {a : Prop} {b : Type} (H1 : a) (H2 : ¬a) : b :=
+definition absurd {a : Prop} {b : Type} (H1 : a) (H2 : ¬a) : b :=
 false.rec b (H2 H1)
 
 /- not -/
diff --git a/library/init/nat.lean b/library/init/nat.lean
index 33d02f251..6033c5fff 100644
--- a/library/init/nat.lean
+++ b/library/init/nat.lean
@@ -196,7 +196,7 @@ namespace nat
     (λh, inl (le_succ_of_le h))
     (λh, elim (eq_or_lt_of_le h) (λe, inl (eq.subst e !le.refl)) inr)) b
 
-  theorem lt_ge_by_cases {a b : ℕ} {P : Type} (H1 : a < b → P) (H2 : a ≥ b → P) : P :=
+  definition lt_ge_by_cases {a b : ℕ} {P : Type} (H1 : a < b → P) (H2 : a ≥ b → P) : P :=
   by_cases H1 (λh, H2 (elim !lt_or_ge (λa, absurd a h) (λa, a)))
 
   definition lt.by_cases {a b : ℕ} {P : Type} (H1 : a < b → P) (H2 : a = b → P) (H3 : b < a → P) : P :=
diff --git a/library/logic/axioms/examples/diaconescu.lean b/library/logic/axioms/examples/diaconescu.lean
index 29f316432..0fa6956e8 100644
--- a/library/logic/axioms/examples/diaconescu.lean
+++ b/library/logic/axioms/examples/diaconescu.lean
@@ -18,8 +18,8 @@ parameter  p : Prop
 private definition U (x : Prop) : Prop := x = true ∨ p
 private definition V (x : Prop) : Prop := x = false ∨ p
 
-private definition u := epsilon U
-private definition v := epsilon V
+private noncomputable definition u := epsilon U
+private noncomputable definition v := epsilon V
 
 private lemma u_def : U u :=
 epsilon_spec (exists.intro true (or.inl rfl))
diff --git a/library/logic/axioms/examples/leftinv_of_inj.lean b/library/logic/axioms/examples/leftinv_of_inj.lean
index 3d4cb153a..d90ffe431 100644
--- a/library/logic/axioms/examples/leftinv_of_inj.lean
+++ b/library/logic/axioms/examples/leftinv_of_inj.lean
@@ -12,7 +12,7 @@ with (∃ a : A, f a = b).
 import logic.axioms.classical
 open function
 
-definition mk_left_inv {A B : Type} [h : nonempty A] (f : A → B) : B → A :=
+noncomputable definition mk_left_inv {A B : Type} [h : nonempty A] (f : A → B) : B → A :=
 λ b : B, if ex : (∃ a : A, f a = b) then some ex else inhabited.value (inhabited_of_nonempty h)
 
 theorem has_left_inverse_of_injective {A B : Type} {f : A → B} : nonempty A → injective f → has_left_inverse f :=
diff --git a/library/logic/axioms/hilbert.lean b/library/logic/axioms/hilbert.lean
index 70eaa0aa0..96752cbee 100644
--- a/library/logic/axioms/hilbert.lean
+++ b/library/logic/axioms/hilbert.lean
@@ -28,7 +28,7 @@ inhabited_of_nonempty (obtain w Hw, from H, nonempty.intro w)
 
 /- the Hilbert epsilon function -/
 
-definition epsilon {A : Type} [H : nonempty A] (P : A → Prop) : A :=
+noncomputable definition epsilon {A : Type} [H : nonempty A] (P : A → Prop) : A :=
 let u : {x | (∃y, P y) → P x} :=
   strong_indefinite_description P H in
 elt_of u
@@ -47,7 +47,7 @@ epsilon_spec_aux (nonempty_of_exists Hex) P Hex
 theorem epsilon_singleton {A : Type} (a : A) : @epsilon A (nonempty.intro a) (λx, x = a) = a :=
 epsilon_spec (exists.intro a (eq.refl a))
 
-definition some {A : Type} {P : A → Prop} (H : ∃x, P x) : A :=
+noncomputable definition some {A : Type} {P : A → Prop} (H : ∃x, P x) : A :=
 @epsilon A (nonempty_of_exists H) P
 
 theorem some_spec {A : Type} {P : A → Prop} (H : ∃x, P x) : P (some H) :=
diff --git a/library/logic/examples/colog88.lean b/library/logic/examples/colog88.lean
index 1532a9ce2..55e450283 100644
--- a/library/logic/examples/colog88.lean
+++ b/library/logic/examples/colog88.lean
@@ -32,7 +32,7 @@ axiom recA   : Π {C : A → Type}, (Π (p : Phi A), C (introA p)) → (Π (a :
 axiom recA_comp : Π {C : A → Type} (H : Π (p : Phi A), C (introA p)) (p : Phi A), recA H (introA p) = H p
 
 -- The recursor could be used to define matchA
-definition matchA (a : A) : Phi A :=
+noncomputable definition matchA (a : A) : Phi A :=
 recA (λ p, p) a
 
 -- and the computation rule would allows us to define
@@ -57,7 +57,7 @@ lemma i_injective {T : Type} {a b : T} : i a = i b → a = b :=
   eq.symm e₃
 
 -- Hence, by composition, we get an injection f from (A → Prop) to A
-definition f : (A → Prop) → A :=
+noncomputable definition f : (A → Prop) → A :=
 λ p, introA (i p)
 
 lemma f_injective : ∀ {p p' : A → Prop}, f p = f p' → p = p':=
@@ -68,11 +68,11 @@ lemma f_injective : ∀ {p p' : A → Prop}, f p = f p' → p = p':=
   We are now back to the usual Cantor-Russel paradox.
   We can define
 -/
-definition P0 (a : A) : Prop :=
+noncomputable definition P0 (a : A) : Prop :=
 ∃ (P : A → Prop), f P = a ∧ ¬ P a
 -- i.e., P0 a := codes a set P such that x∉P
 
-definition x0 : A := f P0
+noncomputable definition x0 : A := f P0
 
 lemma fP0_eq : f P0 = x0 :=
 rfl
diff --git a/library/logic/examples/negative.lean b/library/logic/examples/negative.lean
index 07da4775b..2e8d07afe 100644
--- a/library/logic/examples/negative.lean
+++ b/library/logic/examples/negative.lean
@@ -23,8 +23,8 @@ axiom introD : (D → D) → D
 axiom recD   : Π {C : D → Type}, (Π (f : D → D) (r : Π d, C (f d)), C (introD f)) → (Π (d : D), C d)
 -- We would also get a computational rule for the eliminator, but we don't need it for deriving the inconsistency.
 
-definition id : D → D := λd, d
-definition v  : D     := introD id
+noncomputable definition id : D → D := λd, d
+noncomputable definition v  : D     := introD id
 
 theorem inconsistent : false :=
 recD (λ f ih, ih v) v
diff --git a/library/logic/instances.lean b/library/logic/instances.lean
index b8e89740c..8c3547ffc 100644
--- a/library/logic/instances.lean
+++ b/library/logic/instances.lean
@@ -35,6 +35,8 @@ is_congruence2.mk @congr_imp
 theorem is_congruence_iff : is_congruence2 iff iff iff iff :=
 is_congruence2.mk @congr_iff
 
+reveal is_congruence_not is_congruence_and is_congruence_or is_congruence_imp is_congruence_iff
+
 definition is_congruence_not_compose [instance] := is_congruence.compose is_congruence_not
 definition is_congruence_and_compose [instance] := is_congruence.compose21 is_congruence_and
 definition is_congruence_or_compose [instance] := is_congruence.compose21 is_congruence_or
diff --git a/src/emacs/lean-syntax.el b/src/emacs/lean-syntax.el
index 282835abc..111d9b8c8 100644
--- a/src/emacs/lean-syntax.el
+++ b/src/emacs/lean-syntax.el
@@ -8,7 +8,7 @@
 (require 'rx)
 
 (defconst lean-keywords
-  '("import" "prelude" "tactic_hint" "protected" "private" "definition" "renaming"
+  '("import" "prelude" "tactic_hint" "protected" "private" "noncomputable" "definition" "renaming"
     "hiding" "exposing" "parameter" "parameters" "begin" "begin+" "proof" "qed" "conjecture" "constant" "constants"
     "hypothesis" "lemma" "corollary" "variable" "variables" "premise" "premises"
     "print" "theorem" "example" "abbreviation" "abstract"
diff --git a/src/frontends/lean/decl_cmds.cpp b/src/frontends/lean/decl_cmds.cpp
index 97a6dd3a1..af3bb7cda 100644
--- a/src/frontends/lean/decl_cmds.cpp
+++ b/src/frontends/lean/decl_cmds.cpp
@@ -16,6 +16,7 @@ Author: Leonardo de Moura
 #include "library/aliases.h"
 #include "library/private.h"
 #include "library/protected.h"
+#include "library/noncomputable.h"
 #include "library/placeholder.h"
 #include "library/locals.h"
 #include "library/explicit.h"
@@ -630,6 +631,7 @@ class definition_cmd_fn {
     def_cmd_kind      m_kind;
     bool              m_is_private;
     bool              m_is_protected;
+    bool              m_is_noncomputable;
     pos_info          m_pos;
 
     name              m_name;
@@ -864,6 +866,16 @@ class definition_cmd_fn {
 
     void register_decl(name const & n, name const & real_n, expr const & type) {
         if (m_kind != Example) {
+            if (!m_is_noncomputable && is_marked_noncomputable(m_env, real_n)) {
+                auto reason = get_noncomputable_reason(m_env, real_n);
+                lean_assert(reason);
+                if (m_p.in_theorem_queue(*reason)) {
+                    throw exception(sstream() << "definition '" << n << "' was marked as noncomputable because '" << *reason
+                                    << "' is still in theorem queue (solution: use command 'reveal " << *reason << "'");
+                } else {
+                    throw exception(sstream() << "definition '" << n << "' is noncomputable, it depends on '" << *reason << "'");
+                }
+            }
             // TODO(Leo): register aux_decls
             if (!m_is_private)
                 m_p.add_decl_index(real_n, m_pos, m_p.get_cmd_token(), type);
@@ -995,7 +1007,7 @@ class definition_cmd_fn {
                 m_ls = append(m_ls, new_ls);
                 m_type = postprocess(m_env, m_type);
                 expr type_as_is = m_p.save_pos(mk_as_is(m_type), type_pos);
-                if (!m_p.collecting_info() && m_kind == Theorem && m_p.num_threads() > 1) {
+                if (!m_p.collecting_info() && !m_is_noncomputable && m_kind == Theorem && m_p.num_threads() > 1) {
                     // Add as axiom, and create a task to prove the theorem.
                     // Remark: we don't postpone the "proof" of Examples.
                     m_p.add_delayed_theorem(m_env, m_real_name, m_ls, type_as_is, m_value);
@@ -1053,9 +1065,9 @@ class definition_cmd_fn {
     }
 
 public:
-    definition_cmd_fn(parser & p, def_cmd_kind kind, bool is_private, bool is_protected):
+    definition_cmd_fn(parser & p, def_cmd_kind kind, bool is_private, bool is_protected, bool is_noncomputable):
         m_p(p), m_env(m_p.env()), m_kind(kind),
-        m_is_private(is_private), m_is_protected(is_protected),
+        m_is_private(is_private), m_is_protected(is_protected), m_is_noncomputable(is_noncomputable),
         m_pos(p.pos()), m_attributes(kind == Abbreviation || kind == LocalAbbreviation) {
         lean_assert(!(m_is_private && m_is_protected));
     }
@@ -1082,26 +1094,31 @@ public:
     }
 };
 
-static environment definition_cmd_core(parser & p, def_cmd_kind kind, bool is_private, bool is_protected) {
-    return definition_cmd_fn(p, kind, is_private, is_protected)();
+static environment definition_cmd_core(parser & p, def_cmd_kind kind, bool is_private, bool is_protected, bool is_noncomputable) {
+    return definition_cmd_fn(p, kind, is_private, is_protected, is_noncomputable)();
 }
 static environment definition_cmd(parser & p) {
-    return definition_cmd_core(p, Definition, false, false);
+    return definition_cmd_core(p, Definition, false, false, false);
 }
 static environment abbreviation_cmd(parser & p) {
-    return definition_cmd_core(p, Abbreviation, false, false);
+    return definition_cmd_core(p, Abbreviation, false, false, false);
 }
 environment local_abbreviation_cmd(parser & p) {
-    return definition_cmd_core(p, LocalAbbreviation, true, false);
+    return definition_cmd_core(p, LocalAbbreviation, true, false, false);
 }
 static environment theorem_cmd(parser & p) {
-    return definition_cmd_core(p, Theorem, false, false);
+    return definition_cmd_core(p, Theorem, false, false, false);
 }
 static environment example_cmd(parser & p) {
-    definition_cmd_core(p, Example, false, false);
+    definition_cmd_core(p, Example, false, false, false);
     return p.env();
 }
 static environment private_definition_cmd(parser & p) {
+    bool is_noncomputable = false;
+    if (p.curr_is_token(get_noncomputable_tk())) {
+        is_noncomputable = true;
+        p.next();
+    }
     def_cmd_kind kind = Definition;
     if (p.curr_is_token_or_id(get_definition_tk())) {
         p.next();
@@ -1114,7 +1131,7 @@ static environment private_definition_cmd(parser & p) {
     } else {
         throw parser_error("invalid 'private' definition/theorem, 'definition' or 'theorem' expected", p.pos());
     }
-    return definition_cmd_core(p, kind, true, false);
+    return definition_cmd_core(p, kind, true, false, is_noncomputable);
 }
 static environment protected_definition_cmd(parser & p) {
     if (p.curr_is_token_or_id(get_axiom_tk())) {
@@ -1130,6 +1147,11 @@ static environment protected_definition_cmd(parser & p) {
         p.next();
         return variables_cmd_core(p, variable_kind::Constant, true);
     } else {
+        bool is_noncomputable = false;
+        if (p.curr_is_token(get_noncomputable_tk())) {
+            is_noncomputable = true;
+            p.next();
+        }
         def_cmd_kind kind = Definition;
         if (p.curr_is_token_or_id(get_definition_tk())) {
             p.next();
@@ -1142,9 +1164,33 @@ static environment protected_definition_cmd(parser & p) {
         } else {
             throw parser_error("invalid 'protected' definition/theorem, 'definition' or 'theorem' expected", p.pos());
         }
-        return definition_cmd_core(p, kind, false, true);
+        return definition_cmd_core(p, kind, false, true, is_noncomputable);
     }
 }
+static environment noncomputable_cmd(parser & p) {
+    bool is_private   = false;
+    bool is_protected = false;
+    if (p.curr_is_token(get_private_tk())) {
+        is_private = true;
+        p.next();
+    } else if (p.curr_is_token(get_protected_tk())) {
+        is_protected = true;
+        p.next();
+    }
+    def_cmd_kind kind = Definition;
+    if (p.curr_is_token_or_id(get_definition_tk())) {
+        p.next();
+    } else if (p.curr_is_token_or_id(get_abbreviation_tk())) {
+        p.next();
+        kind = Abbreviation;
+    } else if (p.curr_is_token_or_id(get_theorem_tk())) {
+        p.next();
+        kind       = Theorem;
+    } else {
+        throw parser_error("invalid 'noncomputable' definition/theorem, 'definition' or 'theorem' expected", p.pos());
+    }
+    return definition_cmd_core(p, kind, is_private, is_protected, true);
+}
 
 static environment include_cmd_core(parser & p, bool include) {
     if (!p.curr_is_identifier())
@@ -1222,6 +1268,7 @@ void register_decl_cmds(cmd_table & r) {
     add_cmd(r, cmd_info("constants",    "declare new constants (aka top-level variables)", constants_cmd));
     add_cmd(r, cmd_info("axioms",       "declare new axioms", axioms_cmd));
     add_cmd(r, cmd_info("definition",   "add new definition", definition_cmd));
+    add_cmd(r, cmd_info("noncomputable", "add new noncomputable definition", noncomputable_cmd));
     add_cmd(r, cmd_info("example",      "add new example", example_cmd));
     add_cmd(r, cmd_info("private",      "add new private definition/theorem", private_definition_cmd));
     add_cmd(r, cmd_info("protected",    "add new protected definition/theorem", protected_definition_cmd));
diff --git a/src/frontends/lean/token_table.cpp b/src/frontends/lean/token_table.cpp
index b83b5713e..fb1beac26 100644
--- a/src/frontends/lean/token_table.cpp
+++ b/src/frontends/lean/token_table.cpp
@@ -105,7 +105,7 @@ void init_token_table(token_table & t) {
 
     char const * commands[] =
         {"theorem", "axiom", "axioms", "variable", "protected", "private", "reveal",
-         "definition", "example", "coercion", "abbreviation",
+         "definition", "example", "coercion", "abbreviation", "noncomputable",
          "variables", "parameter", "parameters", "constant", "constants", "[persistent]", "[visible]", "[instance]", "[trans-instance]",
          "[none]", "[class]", "[coercion]", "[reducible]", "[irreducible]", "[semireducible]", "[quasireducible]",
          "[simp]", "[congr]", "[parsing-only]", "[multiple-instances]", "[symm]", "[trans]", "[refl]", "[subst]", "[recursor",
diff --git a/src/frontends/lean/tokens.cpp b/src/frontends/lean/tokens.cpp
index a207a8d83..e05fbd198 100644
--- a/src/frontends/lean/tokens.cpp
+++ b/src/frontends/lean/tokens.cpp
@@ -88,6 +88,7 @@ static name const * g_begin_tk = nullptr;
 static name const * g_begin_plus_tk = nullptr;
 static name const * g_end_tk = nullptr;
 static name const * g_private_tk = nullptr;
+static name const * g_protected_tk = nullptr;
 static name const * g_definition_tk = nullptr;
 static name const * g_theorem_tk = nullptr;
 static name const * g_abbreviation_tk = nullptr;
@@ -151,6 +152,7 @@ static name const * g_trust_tk = nullptr;
 static name const * g_metaclasses_tk = nullptr;
 static name const * g_inductive_tk = nullptr;
 static name const * g_this_tk = nullptr;
+static name const * g_noncomputable_tk = nullptr;
 void initialize_tokens() {
     g_period_tk = new name{"."};
     g_placeholder_tk = new name{"_"};
@@ -237,6 +239,7 @@ void initialize_tokens() {
     g_begin_plus_tk = new name{"begin+"};
     g_end_tk = new name{"end"};
     g_private_tk = new name{"private"};
+    g_protected_tk = new name{"protected"};
     g_definition_tk = new name{"definition"};
     g_theorem_tk = new name{"theorem"};
     g_abbreviation_tk = new name{"abbreviation"};
@@ -300,6 +303,7 @@ void initialize_tokens() {
     g_metaclasses_tk = new name{"metaclasses"};
     g_inductive_tk = new name{"inductive"};
     g_this_tk = new name{"this"};
+    g_noncomputable_tk = new name{"noncomputable"};
 }
 void finalize_tokens() {
     delete g_period_tk;
@@ -387,6 +391,7 @@ void finalize_tokens() {
     delete g_begin_plus_tk;
     delete g_end_tk;
     delete g_private_tk;
+    delete g_protected_tk;
     delete g_definition_tk;
     delete g_theorem_tk;
     delete g_abbreviation_tk;
@@ -450,6 +455,7 @@ void finalize_tokens() {
     delete g_metaclasses_tk;
     delete g_inductive_tk;
     delete g_this_tk;
+    delete g_noncomputable_tk;
 }
 name const & get_period_tk() { return *g_period_tk; }
 name const & get_placeholder_tk() { return *g_placeholder_tk; }
@@ -536,6 +542,7 @@ name const & get_begin_tk() { return *g_begin_tk; }
 name const & get_begin_plus_tk() { return *g_begin_plus_tk; }
 name const & get_end_tk() { return *g_end_tk; }
 name const & get_private_tk() { return *g_private_tk; }
+name const & get_protected_tk() { return *g_protected_tk; }
 name const & get_definition_tk() { return *g_definition_tk; }
 name const & get_theorem_tk() { return *g_theorem_tk; }
 name const & get_abbreviation_tk() { return *g_abbreviation_tk; }
@@ -599,4 +606,5 @@ name const & get_trust_tk() { return *g_trust_tk; }
 name const & get_metaclasses_tk() { return *g_metaclasses_tk; }
 name const & get_inductive_tk() { return *g_inductive_tk; }
 name const & get_this_tk() { return *g_this_tk; }
+name const & get_noncomputable_tk() { return *g_noncomputable_tk; }
 }
diff --git a/src/frontends/lean/tokens.h b/src/frontends/lean/tokens.h
index 0c7f6e235..6ea041b5e 100644
--- a/src/frontends/lean/tokens.h
+++ b/src/frontends/lean/tokens.h
@@ -90,6 +90,7 @@ name const & get_begin_tk();
 name const & get_begin_plus_tk();
 name const & get_end_tk();
 name const & get_private_tk();
+name const & get_protected_tk();
 name const & get_definition_tk();
 name const & get_theorem_tk();
 name const & get_abbreviation_tk();
@@ -153,4 +154,5 @@ name const & get_trust_tk();
 name const & get_metaclasses_tk();
 name const & get_inductive_tk();
 name const & get_this_tk();
+name const & get_noncomputable_tk();
 }
diff --git a/src/frontends/lean/tokens.txt b/src/frontends/lean/tokens.txt
index 0d731b9ea..f1b4e3aec 100644
--- a/src/frontends/lean/tokens.txt
+++ b/src/frontends/lean/tokens.txt
@@ -83,6 +83,7 @@ begin        begin
 begin_plus   begin+
 end          end
 private      private
+protected    protected
 definition   definition
 theorem      theorem
 abbreviation abbreviation
@@ -146,3 +147,4 @@ trust        trust
 metaclasses  metaclasses
 inductive    inductive
 this         this
+noncomputable noncomputable
diff --git a/src/library/module.cpp b/src/library/module.cpp
index 792354ed3..71e3fc98c 100644
--- a/src/library/module.cpp
+++ b/src/library/module.cpp
@@ -23,6 +23,7 @@ Author: Leonardo de Moura
 #include "kernel/quotient/quotient.h"
 #include "kernel/hits/hits.h"
 #include "library/module.h"
+#include "library/noncomputable.h"
 #include "library/sorry.h"
 #include "library/kernel_serializer.h"
 #include "library/unfold_macros.h"
@@ -219,11 +220,15 @@ static environment export_decl(environment const & env, declaration const & d) {
 environment add(environment const & env, certified_declaration const & d) {
     environment new_env = env.add(d);
     declaration _d = d.get_declaration();
+    if (!check_computable(new_env, _d.get_name()))
+        new_env = mark_noncomputable(new_env, _d.get_name());
     return export_decl(update_module_defs(new_env, _d), _d);
 }
 
 environment add(environment const & env, declaration const & d) {
     environment new_env = env.add(d);
+    if (!check_computable(new_env, d.get_name()))
+        new_env = mark_noncomputable(new_env, d.get_name());
     return export_decl(update_module_defs(new_env, d), d);
 }
 
diff --git a/src/library/noncomputable.cpp b/src/library/noncomputable.cpp
index c52138ea1..a0635fea0 100644
--- a/src/library/noncomputable.cpp
+++ b/src/library/noncomputable.cpp
@@ -62,6 +62,11 @@ bool is_noncomputable(environment const & env, name const & n) {
     return is_noncomputable(tc, ext, n);
 }
 
+bool is_marked_noncomputable(environment const & env, name const & n) {
+    auto ext = get_extension(env);
+    return ext.m_noncomputable.contains(n);
+}
+
 environment mark_noncomputable(environment const & env, name const & n) {
     auto ext = get_extension(env);
     ext.m_noncomputable.insert(n);
@@ -69,23 +74,29 @@ environment mark_noncomputable(environment const & env, name const & n) {
     return module::add(new_env, *g_key, [=](environment const &, serializer & s) { s << n; });
 }
 
-void validate_computable_definition(environment const & env, name const & n) {
+optional<name> get_noncomputable_reason(environment const & env, name const & n) {
     if (!is_standard(env))
-        return; // do nothing if it is not a standard kernel
-    type_checker tc(env);
+        return optional<name>(); // do nothing if it is not a standard kernel
     declaration const & d = env.get(n);
     if (!d.is_definition())
-        return;
+        return optional<name>();
+    type_checker tc(env);
     if (tc.is_prop(d.get_type()).first)
-        return; // definition is a proposition, then do nothing
+        return optional<name>(); // definition is a proposition, then do nothing
     expr const & v = d.get_value();
     auto ext = get_extension(env);
+    optional<name> r;
     for_each(v, [&](expr const & e, unsigned) {
             if (is_constant(e) && is_noncomputable(tc, ext, const_name(e))) {
-                throw exception(sstream() << "definition '" << n << "' is noncomputable, it depends on '" << const_name(e) << "'");
+                r = const_name(e);
             }
             return true;
         });
+    return r;
+}
+
+bool check_computable(environment const & env, name const & n) {
+    return !get_noncomputable_reason(env, n);
 }
 
 void initialize_noncomputable() {
diff --git a/src/library/noncomputable.h b/src/library/noncomputable.h
index e21f91297..d0cc71eae 100644
--- a/src/library/noncomputable.h
+++ b/src/library/noncomputable.h
@@ -8,12 +8,14 @@ Author: Leonardo de Moura
 #include "kernel/environment.h"
 
 namespace lean {
+bool is_marked_noncomputable(environment const & env, name const & n);
 /** \brief Return true if \c n is noncomputable */
 bool is_noncomputable(environment const & env, name const & n);
 /** \brief Mark \c n as noncomputable */
 environment mark_noncomputable(environment const & env, name const & n);
-/** \brief Throw an exception if \c n depends on a noncomputable definition */
-void validate_computable_definition(environment const & env, name const & n);
+/** \brief In standard mode, check if definitions that are not propositions can compute */
+bool check_computable(environment const & env, name const & n);
+optional<name> get_noncomputable_reason(environment const & env, name const & n);
 void initialize_noncomputable();
 void finalize_noncomputable();
 }
diff --git a/tests/lean/empty.lean b/tests/lean/empty.lean
index d0e7e2f80..e2198dc72 100644
--- a/tests/lean/empty.lean
+++ b/tests/lean/empty.lean
@@ -1,6 +1,6 @@
 import logic logic.axioms.hilbert
 open inhabited nonempty
 
-definition v1 : Prop := epsilon (λ x, true)
+noncomputable definition v1 : Prop := epsilon (λ x, true)
 inductive Empty : Type
-definition v2 : Empty := epsilon (λ x, true)
+noncomputable definition v2 : Empty := epsilon (λ x, true)
diff --git a/tests/lean/empty.lean.expected.out b/tests/lean/empty.lean.expected.out
index e09a329ae..265bbd604 100644
--- a/tests/lean/empty.lean.expected.out
+++ b/tests/lean/empty.lean.expected.out
@@ -1,3 +1,3 @@
-empty.lean:6:25: error: failed to synthesize placeholder
+empty.lean:6:39: error: failed to synthesize placeholder
 
 ⊢ nonempty Empty
diff --git a/tests/lean/errors.lean b/tests/lean/errors.lean
index 591cf06b1..18a8d78e9 100644
--- a/tests/lean/errors.lean
+++ b/tests/lean/errors.lean
@@ -30,7 +30,7 @@ end foo
 
 open nat
 
-definition bla : nat :=
+noncomputable definition bla : nat :=
 foo.tst1 0 0 + foo.tst2 0 0 + foo.tst3 nat 1 1
 
 check foo.tst1
diff --git a/tests/lean/noncomp.lean b/tests/lean/noncomp.lean
new file mode 100644
index 000000000..62bef47d7
--- /dev/null
+++ b/tests/lean/noncomp.lean
@@ -0,0 +1,8 @@
+open nat
+
+axiom n : nat
+
+definition f (x : nat) :=  -- Error this is not computable
+x + n
+
+noncomputable definition f (x : nat) := x + n
diff --git a/tests/lean/noncomp.lean.expected.out b/tests/lean/noncomp.lean.expected.out
new file mode 100644
index 000000000..f35dfadde
--- /dev/null
+++ b/tests/lean/noncomp.lean.expected.out
@@ -0,0 +1 @@
+noncomp.lean:5:0: error: definition 'f' is noncomputable, it depends on 'n'
diff --git a/tests/lean/run/fibrant_class1.lean b/tests/lean/run/fibrant_class1.lean
index 914bc35d3..3c231cd0c 100644
--- a/tests/lean/run/fibrant_class1.lean
+++ b/tests/lean/run/fibrant_class1.lean
@@ -12,5 +12,5 @@ attribute path_fibrant [instance]
 axiom imp_fibrant {A : Type'} {B : Type'} [C1 : fibrant A] [C2 : fibrant B] : fibrant (A → B)
 attribute imp_fibrant [instance]
 
-definition test {A : Type} [fA : fibrant A] {x y : A} :
+noncomputable definition test {A : Type} [fA : fibrant A] {x y : A} :
 Π (z : A), y ≈ z → fibrant (x ≈ y → x ≈ z) := _
diff --git a/tests/lean/run/fibrant_class2.lean b/tests/lean/run/fibrant_class2.lean
index b84da99e4..59fffb7c8 100644
--- a/tests/lean/run/fibrant_class2.lean
+++ b/tests/lean/run/fibrant_class2.lean
@@ -13,20 +13,20 @@ attribute path_fibrant [instance]
 
 notation a ≈ b := path a b
 
-definition test {A : Type} [fA : fibrant A] {x y : A} :
+noncomputable definition test {A : Type} [fA : fibrant A] {x y : A} :
   Π (z : A), y ≈ z → fibrant (x ≈ y → x ≈ z) := take z p, _
 
-definition test2 {A : Type} [fA : fibrant A] {x y : A} :
+noncomputable definition test2 {A : Type} [fA : fibrant A] {x y : A} :
 Π (z : A), y ≈ z → fibrant (x ≈ y → x ≈ z) := _
 
-definition test3 {A : Type} [fA : fibrant A] {x y : A} :
+noncomputable definition test3 {A : Type} [fA : fibrant A] {x y : A} :
   Π (z : A), y ≈ z → fibrant (x ≈ z) := _
 
-definition test4 {A : Type} [fA : fibrant A] {x y z : A} :
+noncomputable definition test4 {A : Type} [fA : fibrant A] {x y z : A} :
   fibrant (x ≈ y → x ≈ z) := _
 
 axiom imp_fibrant {A : Type} {B : Type} [C1 : fibrant A] [C2 : fibrant B] : fibrant (A → B)
 attribute imp_fibrant [instance]
 
-definition test5 {A : Type} [fA : fibrant A] {x y : A} :
+noncomputable definition test5 {A : Type} [fA : fibrant A] {x y : A} :
 Π (z : A), y ≈ z → fibrant (x ≈ y → x ≈ z) := _
diff --git a/tests/lean/run/tactic26.lean b/tests/lean/run/tactic26.lean
index 70e229eba..6bad0664a 100644
--- a/tests/lean/run/tactic26.lean
+++ b/tests/lean/run/tactic26.lean
@@ -14,7 +14,7 @@ theorem inr_inhabited (A : Type) {B : Type} (H : inhabited B) : inhabited (sum A
 
 notation `(` h `|` r:(foldl `|` (e r, tactic.or_else r e) h) `)` := r
 infixl `;`:15 := tactic.and_then
-
+reveal inl_inhabited inr_inhabited
 definition my_tac := fixpoint (λ t, ( apply @inl_inhabited; t
                                     | apply @inr_inhabited; t
                                     | apply @num.is_inhabited
diff --git a/tests/lean/run/tactic28.lean b/tests/lean/run/tactic28.lean
index 0f1e51c7f..27c1104da 100644
--- a/tests/lean/run/tactic28.lean
+++ b/tests/lean/run/tactic28.lean
@@ -16,7 +16,7 @@ infixl `..`:10 := append
 
 notation `(` h `|` r:(foldl `|` (e r, tactic.or_else r e) h) `)` := r
 infixl `;`:15 := tactic.and_then
-
+reveal inl_inhabited inr_inhabited
 definition my_tac := repeat (trace "iteration"; state;
                               (  apply @inl_inhabited; trace "used inl"
                               .. apply @inr_inhabited; trace "used inr"
diff --git a/tests/lean/slow/path_groupoids.lean b/tests/lean/slow/path_groupoids.lean
index 540b86ef3..a0ce90a0a 100644
--- a/tests/lean/slow/path_groupoids.lean
+++ b/tests/lean/slow/path_groupoids.lean
@@ -391,10 +391,12 @@ theorem triple_induction
   C x y z w p q r :=
 path.rec_on p (take z q, path.rec_on q (take w r, path.rec_on r H)) z q w r
 
+reveal path.double_induction2
 -- try this again
 definition concat_pV_p_new {A : Type} {x y z : A} (p : x ≈ z) (q : y ≈ z) : (p ⬝ q**) ⬝ q ≈ p :=
 double_induction2 p q idp
 
+reveal path.triple_induction
 definition transport_p_pp {A : Type} (P : A → Type)
      {x y z w : A} (p : x ≈ y) (q : y ≈ z) (r : z ≈ w) (u : P x) :
    ap (λe, e # u) (concat_p_pp p q r) ⬝ (transport_pp P (p ⬝ q) r u) ⬝