diff --git a/README.md b/README.md
index a6ac9dda0..2621bc195 100644
--- a/README.md
+++ b/README.md
@@ -28,6 +28,28 @@ Then run:
 npm run dev
 ```
 
+### ⚙️ Configuration
+
+To configure Astro, add a `astro.config.mjs` file in the root of your project. All of the options can be omitted. Here are the defaults:
+
+```js
+export default {
+  /** Where to resolve all URLs relative to. Useful if you have a monorepo project. */
+  projectRoot: '.',
+  /** Path to Astro components, pages, and data */
+  astroRoot: './astro',
+  /** When running `astro build`, path to final static output */
+  dist: './_site',
+  /** A folder of static files Astro will copy to the root. Useful for favicons, images, and other files that don‘t need processing. */
+  public: './public',
+  /** Extension-specific handlings */
+  extensions: {
+    /** Set this to "preact" or "react" to determine what *.jsx files should load */
+    '.jsx': 'react',
+  },
+};
+```
+
 ### 💧 Partial Hydration
 
 By default, Astro outputs zero client-side JS. If you'd like to include an interactive component in the client output, you may use any of the following techniques.
@@ -50,7 +72,7 @@ If you‘ve used [Svelte][svelte]’s styles before, Astro works almost the same
 <div class="scoped">I’m a scoped style</div>
 ```
 
-#### Sass
+#### 👓 Sass
 
 Astro also supports [Sass][sass] out-of-the-box; no configuration needed:
 
@@ -71,11 +93,11 @@ Supports:
 - `lang="scss"`: load as the `.scss` extension
 - `lang="sass"`: load as the `.sass` extension (no brackets; indent-style)
 
-#### Autoprefixer
+#### 🦊 Autoprefixer
 
 We also automatically add browser prefixes using [Autoprefixer][autoprefixer]. By default, Astro loads the default values, but you may also specify your own by placing a [Browserslist][browserslist] file in your project root.
 
-#### Tailwind
+#### 🍃 Tailwind
 
 Astro can be configured to use [Tailwind][tailwind] easily! Install the dependencies:
 
diff --git a/src/cli.ts b/src/cli.ts
index e0f0c0dc3..71c9691d7 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -58,14 +58,13 @@ async function printVersion() {
 
 /** Handle `astro run` command */
 async function runCommand(rawRoot: string, cmd: (a: AstroConfig) => Promise<void>) {
-  const astroConfig = await loadConfig(rawRoot);
-  if (typeof astroConfig === 'undefined') {
-    console.error(colors.red('  An astro.config.mjs file is required.\n'));
-    printHelp();
+  try {
+    const astroConfig = await loadConfig(rawRoot);
+    return cmd(astroConfig);
+  } catch (err) {
+    console.error(colors.red(err.toString() || err));
     process.exit(1);
   }
-
-  return cmd(astroConfig);
 }
 
 const cmdMap = new Map([
diff --git a/src/config.ts b/src/config.ts
index 72583df5a..fe4549929 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -2,25 +2,70 @@ import type { AstroConfig } from './@types/astro';
 import { join as pathJoin, resolve as pathResolve } from 'path';
 import { existsSync } from 'fs';
 
+/** Type util */
+const type = (thing: any): string => (Array.isArray(thing) ? 'Array' : typeof thing);
+
+/** Throws error if a user provided an invalid config. Manually-implemented to avoid a heavy validation library. */
+function validateConfig(config: any): void {
+  // basic
+  if (config === undefined || config === null) throw new Error(`[astro config] Config empty!`);
+  if (typeof config !== 'object') throw new Error(`[astro config] Expected object, received ${typeof config}`);
+
+  // strings
+  for (const key of ['projectRoot', 'astroRoot', 'dist', 'public']) {
+    if (config[key] && typeof config[key] !== 'string') {
+      throw new Error(`[astro config] ${key}: ${JSON.stringify(config[key])}\n  Expected string, received ${type(config[key])}.`);
+    }
+  }
+}
+
+/** Set default config values */
+function configDefaults(userConfig?: any): any {
+  const config: any = { ...(userConfig || {}) };
+
+  if (!config.projectRoot) config.projectRoot = '.';
+  if (!config.astroRoot) config.astroRoot = './astro';
+  if (!config.dist) config.dist = './_site';
+  if (!config.public) config.public = './public';
+
+  return config;
+}
+
+/** Turn raw config values into normalized values */
+function normalizeConfig(userConfig: any, root: string): AstroConfig {
+  const config: any = { ...(userConfig || {}) };
+
+  config.projectRoot = new URL(config.projectRoot + '/', root);
+  config.astroRoot = new URL(config.astroRoot + '/', root);
+  config.public = new URL(config.public + '/', root);
+
+  return config as AstroConfig;
+}
+
 /** Attempt to load an `astro.config.mjs` file */
-export async function loadConfig(rawRoot: string | undefined): Promise<AstroConfig | undefined> {
+export async function loadConfig(rawRoot: string | undefined): Promise<AstroConfig> {
   if (typeof rawRoot === 'undefined') {
     rawRoot = process.cwd();
   }
 
+  let config: any;
+
   const root = pathResolve(rawRoot);
   const fileProtocolRoot = `file://${root}/`;
   const astroConfigPath = pathJoin(root, 'astro.config.mjs');
 
-  if (!existsSync(astroConfigPath)) {
-    return undefined;
+  // load
+  if (existsSync(astroConfigPath)) {
+    config = configDefaults((await import(astroConfigPath)).default);
+  } else {
+    config = configDefaults();
   }
 
-  const astroConfig: AstroConfig = (await import(astroConfigPath)).default;
-  astroConfig.projectRoot = new URL(astroConfig.projectRoot + '/', fileProtocolRoot);
-  astroConfig.astroRoot = new URL(astroConfig.astroRoot + '/', fileProtocolRoot);
+  // validate
+  validateConfig(config);
 
-  const publicFolder = astroConfig.public ? astroConfig.public + '/' : './public/';
-  astroConfig.public = new URL(publicFolder, fileProtocolRoot);
-  return astroConfig;
+  // normalize
+  config = normalizeConfig(config, fileProtocolRoot);
+
+  return config as AstroConfig;
 }
diff --git a/test/fixtures/astro-basic/astro.config.mjs b/test/fixtures/astro-basic/astro.config.mjs
deleted file mode 100644
index c7cbdb435..000000000
--- a/test/fixtures/astro-basic/astro.config.mjs
+++ /dev/null
@@ -1,5 +0,0 @@
-export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
-};
diff --git a/test/fixtures/astro-doctype/astro.config.mjs b/test/fixtures/astro-doctype/astro.config.mjs
deleted file mode 100644
index c7cbdb435..000000000
--- a/test/fixtures/astro-doctype/astro.config.mjs
+++ /dev/null
@@ -1,5 +0,0 @@
-export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
-};
diff --git a/test/fixtures/astro-fallback/astro.config.mjs b/test/fixtures/astro-fallback/astro.config.mjs
index ac85e54d8..f50751cfd 100644
--- a/test/fixtures/astro-fallback/astro.config.mjs
+++ b/test/fixtures/astro-fallback/astro.config.mjs
@@ -1,8 +1,5 @@
 export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
   extensions: {
-    '.jsx': 'preact'
-  }
+    '.jsx': 'preact',
+  },
 };
diff --git a/test/fixtures/astro-markdown/astro.config.mjs b/test/fixtures/astro-markdown/astro.config.mjs
index a5bcba200..f50751cfd 100644
--- a/test/fixtures/astro-markdown/astro.config.mjs
+++ b/test/fixtures/astro-markdown/astro.config.mjs
@@ -1,7 +1,4 @@
 export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
   extensions: {
     '.jsx': 'preact',
   },
diff --git a/test/fixtures/astro-request/astro.config.mjs b/test/fixtures/astro-request/astro.config.mjs
deleted file mode 100644
index 48c4c3aad..000000000
--- a/test/fixtures/astro-request/astro.config.mjs
+++ /dev/null
@@ -1,5 +0,0 @@
-export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site'
-};
diff --git a/test/fixtures/astro-styles-ssr/astro.config.mjs b/test/fixtures/astro-styles-ssr/astro.config.mjs
deleted file mode 100644
index c7cbdb435..000000000
--- a/test/fixtures/astro-styles-ssr/astro.config.mjs
+++ /dev/null
@@ -1,5 +0,0 @@
-export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
-};
diff --git a/test/fixtures/react-component/astro.config.mjs b/test/fixtures/react-component/astro.config.mjs
deleted file mode 100644
index 30955cef0..000000000
--- a/test/fixtures/react-component/astro.config.mjs
+++ /dev/null
@@ -1,6 +0,0 @@
-export default {
-  projectRoot: '.',
-  astroRoot: './astro',
-  dist: './_site',
-  // No extensions needed, React is the default.
-};