diff --git a/src/ast.ts b/src/ast.ts
index 36cb678..976d39a 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -146,6 +146,7 @@ export type Definition = {
     line?: number
     column?: number
     start?: number
+    annotation?: Annotation
 }
 
 export type Rebind = {
@@ -157,6 +158,35 @@ export type Rebind = {
     start?: number
 }
 
+export type TypeAST =
+    | { kind: 'type-name', name: string } // Int, String, Bool
+    | { kind: 'type-var', name: string } // a, b
+    | { kind: 'type-function', param: TypeAST, result: TypeAST } // a \ b
+    | { kind: 'type-apply', constructor: TypeAST, args: TypeAST[] } // List a, Maybe Int
+    | { kind: 'type-record', fields: { name: string, type: TypeAST }[] } // { x: Int }
+
+export type TypeConstructor = { name: string, args: TypeAST[] }
+
+export type TypeDefinition = {
+    kind: 'type-definition'
+    name: string
+    params: string[]
+    constructors: TypeConstructor[]
+    line?: number
+    column?: number
+    start?: number
+}
+
+export type Annotation = {
+    constraints: Constraint[]
+    type: TypeAST
+}
+
+export type Constraint = {
+    className: string
+    typeVar: string
+}
+
 export type AST =
     | Literal
     | Variable
@@ -292,7 +322,10 @@ export function prettyPrint(ast: AST, indent = 0): string {
             return `...${prettyPrint(ast.spread, indent)}`;
 
         case 'definition':
-            return `${ast.name} = ${prettyPrint(ast.body, indent)};`;
+            const ann = ast.annotation
+                ? `${ast.name} : ${prettyPrintType(ast.annotation.type)};\n`
+                : '';
+            return `${ann}${ast.name} = ${prettyPrint(ast.body, indent)};`;
 
         default:
             return `Unknown AST kind: ${i}${(ast as any).kind}`
@@ -338,6 +371,44 @@ function prettyPrintPattern(pattern: Pattern): string {
     }
 }
 
+function prettyPrintType(type: TypeAST): string {
+    switch (type.kind) {
+        case 'type-name':
+        case 'type-var':
+            return type.name;
+        case 'type-function':
+            const param = type.param.kind === 'type-function'
+                ? `(${prettyPrintType(type.param)})`
+                : prettyPrintType(type.param);
+            return `${param} \\ ${prettyPrintType(type.result)}`;
+        case 'type-apply':
+            const args = type.args.map(a =>
+                a.kind === 'type-function' || a.kind === 'type-apply'
+                    ? `(${prettyPrintType(a)})`
+                    : prettyPrintType(a)
+            ).join(' ');
+            return `${prettyPrintType(type.constructor)} ${args}`;
+        case 'type-record':
+            const fields = type.fields
+                .map(f => `${f.name} : ${prettyPrintType(f.type)}`)
+                .join(', ');
+            return `{ ${fields} }`;
+    }
+}
+
+function prettyPrintTypeDefinition(td: TypeDefinition): string {
+    const params = td.params.length > 0 ? ' ' + td.params.join(' ') : '';
+    const ctors = td.constructors.map(c => {
+        const args = c.args.map(a =>
+            a.kind === 'type-function' || a.kind === 'type-apply'
+                ? `(${prettyPrintType(a)})`
+                : prettyPrintType(a)
+        ).join(' ');
+        return args ? `${c.name} ${args}` : c.name;
+    }).join(' | ');
+    return `${td.name}${params} = ${ctors};`;
+}
+
 function needsQuotes(key: string): boolean {
     return key === '_' || !/^[a-zA-Z_][a-zA-Z0-9_]*$/.test(key);
 }
diff --git a/src/cg/01-stdlib.cg b/src/cg/01-stdlib.cg
index 0686984..83a97fb 100644
--- a/src/cg/01-stdlib.cg
+++ b/src/cg/01-stdlib.cg
@@ -1,3 +1,6 @@
+Maybe a = None | Some a;
+
+
 # nth : Int \ List a \ Maybe a
 # in host at the moment, until we get typeclasses or something and this can work on strings too
 # nth = i list \ [i, list]
@@ -5,7 +8,7 @@
 #   | [0, [x, ...xs]] \ (Some x)
 #   | [n, [x, ...xs]] \ nth (n - 1) xs;
 
-# map : (a \ b) \ List a \ List b
+map : (a \ b) \ List a \ List b;
 map = f list \ list
   | [] \ []
   | [x, ...xs] \ [f x, ...map f xs];
diff --git a/src/main.ts b/src/main.ts
index 07edb31..a78e982 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -23,7 +23,7 @@ store.viewport = { width: window.innerWidth, height: window.innerHeight };
 try {
     const tokens = tokenize(cgCode);
     const parser = new Parser(tokens, cgCode);
-    const defs = parser.parse();
+    const { definitions: defs } = parser.parse();
     loadDefinitions();
 
     // TODO remove once we're booting from store, files are backup
diff --git a/src/parser.ts b/src/parser.ts
index 1e25974..f1ecc9c 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -1,5 +1,5 @@
 import type { Token } from './lexer'
-import type { AST, MatchCase, Pattern, Definition } from './ast'
+import type { AST, MatchCase, Pattern, Definition, TypeAST, TypeDefinition, TypeConstructor } from './ast'
 import { ParseError } from './error'
 
 export class Parser {
@@ -150,14 +150,35 @@ export class Parser {
         }
     }
 
-    parse(): Definition[] {
+    parse(): { definitions: Definition[], typeDefinitions: TypeDefinition[] } {
         const definitions: Definition[] = [];
+        const typeDefinitions: TypeDefinition[] = [];
 
         while (this.current().kind !== 'eof') {
+            if (this.current().kind === 'type-ident') {
+                typeDefinitions.push(this.parseTypeDefinition());
+                continue;
+            }
+
+            // type annotation
+            if (this.current().kind === 'ident' && this.peek().kind === 'colon') {
+                this.advance(); // eat ident
+
+                this.advance();
+                const type = this.parseType();
+                this.expect('semicolon');
+
+                // parse definition
+                const def = this.parseDefinition();
+                def.annotation = { constraints: [], type };
+
+                definitions.push(def);
+                continue;
+            }
             definitions.push(this.parseDefinition());
         }
 
-        return definitions;
+        return { definitions, typeDefinitions };
     }
 
     private parseDefinition(): Definition {
@@ -545,4 +566,103 @@ export class Parser {
 
         throw ParseError(`Unexpected token: ${token.kind}`, token.line, token.column, this.source);
     }
+
+    private parseTypeAtom(): TypeAST {
+        const token = this.current();
+
+        if (token.kind === 'type-ident') {
+            this.advance();
+            return { kind: 'type-name', name: token.value };
+        }
+
+        if (token.kind === 'ident') {
+            this.advance();
+            return { kind: 'type-var', name: token.value };
+        }
+
+        if (token.kind === 'open-paren') {
+            this.advance();
+            const type = this.parseType();
+            this.expect('close-paren');
+            return type;
+        }
+
+        if (token.kind === 'open-brace') {
+            this.advance();
+            const fields: { name: string, type: TypeAST }[] = [];
+            while (this.current().kind !== 'close-brace') {
+                const name = this.expectIdent() as { value: string };
+                this.expect('colon');
+                const type = this.parseType();
+                fields.push({ name: name.value, type });
+                if (this.current().kind === 'comma') this.advance();
+            }
+            this.expect('close-brace');
+            return { kind: 'type-record', fields };
+        }
+
+        throw ParseError(`Expected type, got ${token.kind}`, token.line, token.column, this.source);
+    }
+
+    private parseTypeApply(): TypeAST {
+        const base = this.parseTypeAtom();
+        const args: TypeAST[] = [];
+        while (this.canStartTypeAtom()) {
+            args.push(this.parseTypeAtom());
+        }
+        if (args.length === 0) return base;
+        return { kind: 'type-apply', constructor: base, args };
+    }
+
+    private canStartTypeAtom(): boolean {
+        const kind = this.current().kind;
+        return kind === 'type-ident' || kind === 'ident' ||
+               kind === 'open-paren' || kind === 'open-brace';
+    }
+
+    private parseType(): TypeAST {
+        const left = this.parseTypeApply();
+
+        if (this.current().kind === 'backslash') {
+            this.advance();
+            const right = this.parseType();
+            return { kind: 'type-function', param: left, result: right };
+        }
+
+        return left;
+    }
+
+    private parseTypeDefinition(): TypeDefinition {
+        const nameToken = this.advance();
+        const name = (nameToken as { value: string }).value;
+
+        // Collect type params
+        const  params: string[] = [];
+        while (this.current().kind === 'ident') {
+            params.push((this.advance() as { value: string }).value);
+        }
+
+        this.expect('equals');
+
+        // Parse constructors separated by |
+        const constructors: TypeConstructor[] = [];
+        constructors.push(this.parseTypeConstructor());
+        while (this.current().kind === 'pipe') {
+            this.advance();
+            constructors.push(this.parseTypeConstructor());
+        }
+
+        if (this.current().kind === 'semicolon') this.advance();
+
+        return { kind: 'type-definition', name, params, constructors, ...this.getPos(nameToken) };
+    }
+
+    private parseTypeConstructor(): TypeConstructor {
+        const name = (this.expect('type-ident') as { value: string }).value;
+        const args: TypeAST[] = [];
+        while (this.canStartTypeAtom()) {
+            args.push(this.parseTypeAtom());
+        }
+        return { name, args };
+    }
 }
diff --git a/src/runtime-js.ts b/src/runtime-js.ts
index db04c8a..6235707 100644
--- a/src/runtime-js.ts
+++ b/src/runtime-js.ts
@@ -209,7 +209,7 @@ export const _rt = {
                 const fullCode = trimmed.endsWith(';') ? trimmed : trimmed + ';';
                 const tokens = tokenize(fullCode);
                 const parser = new Parser(tokens, fullCode);
-                const defs = parser.parse();
+                const { definitions: defs } = parser.parse();
 
                 if (defs.length > 0) {
                     const def = defs[0];
@@ -229,7 +229,7 @@ export const _rt = {
             const wrapped = `_expr = ${trimmed};`;
             const tokens = tokenize(wrapped);
             const parser = new Parser(tokens, wrapped);
-            const defs = parser.parse();
+            const { definitions: defs } = parser.parse();
             const ast = defs[0].body;
 
             // validate free vars
@@ -275,7 +275,7 @@ export function loadDefinitions() {
         for (const [_, source] of Object.entries(saved)) {
             const tokens = tokenize(source as string);
             const parser = new Parser(tokens, source as string);
-            const defs = parser.parse();
+            const { definitions: defs } = parser.parse();
             if (defs.length > 0) {
                 recompile(defs[0].name, defs[0].body);
             }