diff --git a/src/frontends/lean/parser.cpp b/src/frontends/lean/parser.cpp
index d433efa39..de9beb7fe 100644
--- a/src/frontends/lean/parser.cpp
+++ b/src/frontends/lean/parser.cpp
@@ -26,6 +26,7 @@ Author: Leonardo de Moura
 #include "library/module.h"
 #include "library/scoped_ext.h"
 #include "library/num.h"
+#include "library/string.h"
 #include "library/error_handling/error_handling.h"
 #include "frontends/lean/parser.h"
 #include "frontends/lean/parser_bindings.h"
@@ -865,9 +866,15 @@ expr parser::parse_decimal_expr() {
 }
 
 expr parser::parse_string_expr() {
-    // TODO(Leo)
-    next();  // to avoid loop
-    return expr();
+    auto p = pos();
+    std::string v = get_str_val();
+    next();
+    if (!m_has_string)
+        m_has_string = has_string_decls(m_env);
+    if (!*m_has_string)
+        throw parser_error("string cannot be encoded as expression, environment does not contain the type 'string' "
+                           "(solution: use 'import string')", p);
+    return from_string(v);
 }
 
 expr parser::parse_nud() {
diff --git a/src/frontends/lean/parser.h b/src/frontends/lean/parser.h
index ba702f014..b599bd7b6 100644
--- a/src/frontends/lean/parser.h
+++ b/src/frontends/lean/parser.h
@@ -65,6 +65,7 @@ class parser {
     // This flag is when we are trying to parse mutually recursive declarations.
     bool                    m_no_undef_id_error;
     optional<bool>          m_has_num;
+    optional<bool>          m_has_string;
 
     void display_error_pos(unsigned line, unsigned pos);
     void display_error_pos(pos_info p);
diff --git a/src/kernel/expr.h b/src/kernel/expr.h
index 0f1f1a2cb..8c3669b01 100644
--- a/src/kernel/expr.h
+++ b/src/kernel/expr.h
@@ -360,6 +360,7 @@ public:
     bool is_atomic_pp(bool unicode, bool coercion) const { return m_ptr->is_atomic_pp(unicode, coercion); }
     unsigned hash() const { return m_ptr->hash(); }
     void write(serializer & s) const { return m_ptr->write(s); }
+    macro_definition_cell const * raw() const { return m_ptr; }
 };
 
 /** \brief Macro attachments */
diff --git a/src/library/CMakeLists.txt b/src/library/CMakeLists.txt
index 79e004bd3..876b771f6 100644
--- a/src/library/CMakeLists.txt
+++ b/src/library/CMakeLists.txt
@@ -4,7 +4,7 @@ add_library(library deep_copy.cpp expr_lt.cpp io_state.cpp occurs.cpp
   normalize.cpp shared_environment.cpp module.cpp coercion.cpp
   private.cpp placeholder.cpp aliases.cpp level_names.cpp
   update_declaration.cpp choice.cpp scoped_ext.cpp locals.cpp
-  unifier.cpp explicit.cpp num.cpp)
+  unifier.cpp explicit.cpp num.cpp string.cpp)
 # hop_match.cpp)
 
 target_link_libraries(library ${LEAN_LIBS})
diff --git a/src/library/string.cpp b/src/library/string.cpp
new file mode 100644
index 000000000..02335d169
--- /dev/null
+++ b/src/library/string.cpp
@@ -0,0 +1,188 @@
+/*
+Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include <string>
+#include <algorithm>
+#include "kernel/type_checker.h"
+#include "library/kernel_serializer.h"
+#include "library/string.h"
+
+namespace lean {
+static name g_string_macro("string_macro");
+static std::string g_string_opcode("Str");
+
+static expr g_bit(Const(name("bit", "bit")));
+static expr g_b0(Const(name("bit", "b0")));
+static expr g_b1(Const(name("bit", "b1")));
+static expr g_char(Const(name("string", "char")));
+static expr g_ascii(Const(name("string", "ascii")));
+static expr g_string(Const(name("string", "string")));
+static expr g_empty(Const(name("string", "empty")));
+static expr g_str(Const(name("string", "str")));
+
+expr from_string_core(unsigned i, std::string const & s);
+
+/** \brief The string macro is a compact way of encoding strings inside Lean expressions. */
+class string_macro : public macro_definition_cell {
+    std::string m_value;
+public:
+    string_macro(std::string const & v):m_value(v) {}
+    virtual bool lt(macro_definition_cell const & d) const {
+        return m_value < static_cast<string_macro const &>(d).m_value;
+    }
+    virtual name get_name() const { return g_string_macro; }
+    virtual expr get_type(expr const &, expr const *, extension_context &) const {
+        return g_string;
+    }
+    virtual optional<expr> expand(expr const &, extension_context &) const {
+        return some_expr(from_string_core(0, m_value));
+    }
+    virtual unsigned trust_level() const { return 0; }
+    virtual bool operator==(macro_definition_cell const & other) const {
+        string_macro const * other_ptr = dynamic_cast<string_macro const *>(&other);
+        return other_ptr && m_value == other_ptr->m_value;
+    }
+    virtual void display(std::ostream & out) const {
+        out << "\"";
+        for (unsigned i = 0; i < m_value.size(); i++) {
+            char c = m_value[i];
+            if (c == '\n')
+                out << "\\n";
+            else if (c == '\t')
+                out << "\\t";
+            else if (c == '\r')
+                out << "\\r";
+            else if (c == 0)
+                out << "\\0";
+            else if (c == '\"')
+                out << "\\\"";
+            else
+                out << c;
+        }
+        out << "\"";
+    }
+    virtual format pp(formatter const &, options const &) const {
+        std::ostringstream out;
+        display(out);
+        return format(out.str());
+    }
+    virtual bool is_atomic_pp(bool, bool) const { return true; }
+    virtual unsigned hash() const { return std::hash<std::string>()(m_value); }
+    virtual void write(serializer & s) const { s << g_string_opcode << m_value; }
+    std::string const & get_value() const { return m_value; }
+};
+
+expr mk_string_macro(std::string const & v) {
+    return mk_macro(macro_definition(new string_macro(v)));
+}
+
+bool is_string_macro(expr const & e) {
+    return is_macro(e) && dynamic_cast<string_macro const *>(macro_def(e).raw()) != nullptr;
+}
+
+string_macro const & to_string_macro(expr const & e) {
+    lean_assert(is_string_macro(e));
+    return *static_cast<string_macro const *>(macro_def(e).raw());
+}
+
+static register_macro_deserializer_fn
+string_macro_des_fn(g_string_opcode,
+                    [](deserializer & d, unsigned num, expr const *) {
+                         if (num != 0)
+                             throw_corrupted_file();
+                         std::string v = d.read_string();
+                         return mk_string_macro(v);
+                    });
+
+bool has_string_decls(environment const & env) {
+    try {
+        type_checker tc(env);
+        return
+            tc.infer(g_b0)    == g_bit &&
+            tc.infer(g_b1)    == g_bit &&
+            tc.infer(g_ascii) == g_bit >> (g_bit >> (g_bit >> (g_bit >> (g_bit >> (g_bit >> (g_bit >> (g_bit >> g_char))))))) &&
+            tc.infer(g_empty) == g_string &&
+            tc.infer(g_str)   == g_char >> (g_string >> g_string);
+    } catch (...) {
+        return false;
+    }
+}
+
+expr from_char(unsigned char c) {
+    buffer<expr> bits;
+    while (c != 0) {
+        if (c % 2 == 1)
+            bits.push_back(g_b1);
+        else
+            bits.push_back(g_b0);
+        c /= 2;
+    }
+    while (bits.size() < 8)
+        bits.push_back(g_b0);
+    return mk_rev_app(g_ascii, bits.size(), bits.data());
+}
+
+expr from_string_core(unsigned i, std::string const & s) {
+    if (i == s.size())
+        return g_empty;
+    else
+        return mk_app(g_str, from_char(s[i]), from_string_core(i+1, s));
+}
+
+expr from_string(std::string const & s) {
+    return mk_string_macro(s);
+}
+
+bool to_char_core(expr const & e, buffer<char> & tmp) {
+    buffer<expr> args;
+    if (get_app_rev_args(e, args) == g_ascii && args.size() == 8) {
+        unsigned v = 0;
+        for (unsigned i = 0; i < args.size(); i++) {
+            v *= 2;
+            if (args[i] == g_b1)
+                v++;
+            else if (args[i] != g_b0)
+                return false;
+        }
+        tmp.push_back(v);
+        return true;
+    } else {
+        return false;
+    }
+}
+
+bool to_string_core(expr const & e, buffer<char> & tmp) {
+    if (e == g_empty) {
+        return true;
+    } else {
+        buffer<expr> args;
+        return
+            get_app_args(e, args) == g_str &&
+            args.size() == 2 &&
+            to_char_core(args[0], tmp) &&
+            to_string_core(args[1], tmp);
+    }
+}
+
+optional<std::string> to_string(expr const & e) {
+    if (is_string_macro(e)) {
+        return optional<std::string>(to_string_macro(e).get_value());
+    } else {
+        buffer<char> tmp;
+        if (to_string_core(e, tmp)) {
+            std::string r;
+            unsigned i = tmp.size();
+            while (i > 0) {
+                --i;
+                r += tmp[i];
+            }
+            return optional<std::string>(r);
+        } else {
+            return optional<std::string>();
+        }
+    }
+}
+}
diff --git a/src/library/string.h b/src/library/string.h
new file mode 100644
index 000000000..30fdc5c02
--- /dev/null
+++ b/src/library/string.h
@@ -0,0 +1,39 @@
+/*
+Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Leonardo de Moura
+*/
+#include <string>
+#include "kernel/environment.h"
+
+namespace lean {
+/**
+   \brief Return true iff the environment \c env contains the following declarations
+   in the namespace 'bit'
+       b0 : bit
+       b1 : bit
+   and the following ones in the namespace 'string'
+       ascii : bit -> bit -> bit -> bit -> bit -> bit -> bit -> bit -> char
+       empty : string
+       str   : char -> string -> string
+*/
+bool has_string_decls(environment const & env);
+
+/**
+    \brief Return an expression that encodes the given string using the declarations
+    ascii, empty and str.
+
+    \see has_string_decls
+
+    \post *to_string(from_string(s)) == s
+*/
+expr from_string(std::string const & s);
+
+/**
+   \brief If the given expression encodes a string, then convert it back to a string.
+
+   \see from_string
+*/
+optional<std::string> to_string(expr const & e);
+}
diff --git a/tests/lean/run/string.lean b/tests/lean/run/string.lean
new file mode 100644
index 000000000..4768ea11d
--- /dev/null
+++ b/tests/lean/run/string.lean
@@ -0,0 +1,3 @@
+import string
+check "aaa"
+check "B"
\ No newline at end of file