diff --git a/src/common.ts b/src/common.ts
index 638e3cba..af24dd09 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -36,44 +36,46 @@ export enum CommonFlags {
   GET = 1 << 11,
   /** Has a `set` modifier. */
   SET = 1 << 12,
+  /** Has a definite assignment assertion `!` as in `x!: i32;`. */
+  DEFINITE_ASSIGNMENT = 1 << 13,
 
   // Extended modifiers usually derived from basic modifiers
 
   /** Is ambient, that is either declared or nested in a declared element. */
-  AMBIENT = 1 << 13,
+  AMBIENT = 1 << 14,
   /** Is generic. */
-  GENERIC = 1 << 14,
+  GENERIC = 1 << 15,
   /** Is part of a generic context. */
-  GENERIC_CONTEXT = 1 << 15,
+  GENERIC_CONTEXT = 1 << 16,
   /** Is an instance member. */
-  INSTANCE = 1 << 16,
+  INSTANCE = 1 << 17,
   /** Is a constructor. */
-  CONSTRUCTOR = 1 << 17,
+  CONSTRUCTOR = 1 << 18,
   /** Is an arrow function. */
-  ARROW = 1 << 18,
+  ARROW = 1 << 19,
   /** Is a module export. */
-  MODULE_EXPORT = 1 << 19,
+  MODULE_EXPORT = 1 << 20,
   /** Is a module import. */
-  MODULE_IMPORT = 1 << 20,
+  MODULE_IMPORT = 1 << 21,
 
   // Compilation states
 
   /** Is compiled. */
-  COMPILED = 1 << 21,
+  COMPILED = 1 << 22,
   /** Has a constant value and is therefore inlined. */
-  INLINED = 1 << 22,
+  INLINED = 1 << 23,
   /** Is scoped. */
-  SCOPED = 1 << 23,
+  SCOPED = 1 << 24,
   /** Is a trampoline. */
-  TRAMPOLINE = 1 << 24,
+  TRAMPOLINE = 1 << 25,
   /** Is a virtual method. */
-  VIRTUAL = 1 << 25,
+  VIRTUAL = 1 << 26,
   /** Is the main function. */
-  MAIN = 1 << 26,
+  MAIN = 1 << 27,
 
   // Other
 
-  QUOTED = 1 << 27
+  QUOTED = 1 << 28
 }
 
 /** Path delimiter inserted between file system levels. */
diff --git a/src/diagnosticMessages.generated.ts b/src/diagnosticMessages.generated.ts
index fd1f65eb..94af4a26 100644
--- a/src/diagnosticMessages.generated.ts
+++ b/src/diagnosticMessages.generated.ts
@@ -29,6 +29,7 @@ export enum DiagnosticCode {
   Constructor_of_class_0_must_not_require_any_arguments = 216,
   Function_0_cannot_be_inlined_into_itself = 217,
   Cannot_access_method_0_without_calling_it_as_it_requires_this_to_be_set = 218,
+  Optional_properties_are_not_supported = 219,
   Unterminated_string_literal = 1002,
   Identifier_expected = 1003,
   _0_expected = 1005,
@@ -81,6 +82,7 @@ export enum DiagnosticCode {
   Decorators_are_not_valid_here = 1206,
   _abstract_modifier_can_only_appear_on_a_class_method_or_property_declaration = 1242,
   Method_0_cannot_have_an_implementation_because_it_is_marked_abstract = 1245,
+  A_definite_assignment_assertion_is_not_permitted_in_this_context = 1255,
   A_class_may_only_extend_another_class = 1311,
   A_parameter_property_cannot_be_declared_using_a_rest_parameter = 1317,
   Duplicate_identifier_0 = 2300,
@@ -149,6 +151,7 @@ export function diagnosticCodeToString(code: DiagnosticCode): string {
     case 216: return "Constructor of class '{0}' must not require any arguments.";
     case 217: return "Function '{0}' cannot be inlined into itself.";
     case 218: return "Cannot access method '{0}' without calling it as it requires 'this' to be set.";
+    case 219: return "Optional properties are not supported.";
     case 1002: return "Unterminated string literal.";
     case 1003: return "Identifier expected.";
     case 1005: return "'{0}' expected.";
@@ -201,6 +204,7 @@ export function diagnosticCodeToString(code: DiagnosticCode): string {
     case 1206: return "Decorators are not valid here.";
     case 1242: return "'abstract' modifier can only appear on a class, method, or property declaration.";
     case 1245: return "Method '{0}' cannot have an implementation because it is marked abstract.";
+    case 1255: return "A definite assignment assertion '!' is not permitted in this context.";
     case 1311: return "A class may only extend another class.";
     case 1317: return "A parameter property cannot be declared using a rest parameter.";
     case 2300: return "Duplicate identifier '{0}'.";
diff --git a/src/diagnosticMessages.json b/src/diagnosticMessages.json
index 647a172e..b9607098 100644
--- a/src/diagnosticMessages.json
+++ b/src/diagnosticMessages.json
@@ -21,6 +21,7 @@
   "Constructor of class '{0}' must not require any arguments.": 216,
   "Function '{0}' cannot be inlined into itself.": 217,
   "Cannot access method '{0}' without calling it as it requires 'this' to be set.": 218,
+  "Optional properties are not supported.": 219,
 
   "Unterminated string literal.": 1002,
   "Identifier expected.": 1003,
@@ -74,6 +75,7 @@
   "Decorators are not valid here.": 1206,
   "'abstract' modifier can only appear on a class, method, or property declaration.": 1242,
   "Method '{0}' cannot have an implementation because it is marked abstract.": 1245,
+  "A definite assignment assertion '!' is not permitted in this context.": 1255,
   "A class may only extend another class.": 1311,
   "A parameter property cannot be declared using a rest parameter.": 1317,
 
diff --git a/src/extra/ast.ts b/src/extra/ast.ts
index b8cae53a..2e005526 100644
--- a/src/extra/ast.ts
+++ b/src/extra/ast.ts
@@ -990,6 +990,9 @@ export class ASTBuilder {
     this.serializeAccessModifiers(node);
     this.visitIdentifierExpression(node.name);
     var sb = this.sb;
+    if (node.flags & CommonFlags.DEFINITE_ASSIGNMENT) {
+      sb.push("!");
+    }
     var type = node.type;
     if (type) {
       sb.push(": ");
@@ -1376,6 +1379,9 @@ export class ASTBuilder {
     this.visitIdentifierExpression(node.name);
     var type = node.type;
     var sb = this.sb;
+    if (node.flags & CommonFlags.DEFINITE_ASSIGNMENT) {
+      sb.push("!");
+    }
     if (type) {
       sb.push(": ");
       this.visitTypeNode(type);
diff --git a/src/parser.ts b/src/parser.ts
index 5c12524a..3bda5032 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -769,6 +769,9 @@ export class Parser extends DiagnosticEmitter {
     }
     var identifier = Node.createIdentifierExpression(tn.readIdentifier(), tn.range());
     var flags = parentFlags;
+    if (tn.skip(Token.EXCLAMATION)) {
+      flags |= CommonFlags.DEFINITE_ASSIGNMENT;
+    }
 
     var type: CommonTypeNode | null = null;
     if (tn.skip(Token.COLON)) {
@@ -800,13 +803,19 @@ export class Parser extends DiagnosticEmitter {
         ); // recoverable
       }
     }
+    var range = Range.join(identifier.range, tn.range());
+    if ((flags & CommonFlags.DEFINITE_ASSIGNMENT) && initializer) {
+      this.error(
+        DiagnosticCode.A_definite_assignment_assertion_is_not_permitted_in_this_context,
+        range);
+    }
     return Node.createVariableDeclaration(
       identifier,
       type,
       initializer,
       parentDecorators,
       flags,
-      Range.join(identifier.range, tn.range())
+      range
     );
   }
 
@@ -1926,6 +1935,15 @@ export class Parser extends DiagnosticEmitter {
       }
 
       let type: CommonTypeNode | null = null;
+      if (tn.skip(Token.QUESTION)) {
+        this.error(
+          DiagnosticCode.Optional_properties_are_not_supported,
+          tn.range(startPos, tn.pos)
+        );
+      }
+      if (tn.skip(Token.EXCLAMATION)) {
+        flags |= CommonFlags.DEFINITE_ASSIGNMENT;
+      }
       if (tn.skip(Token.COLON)) {
         type = this.parseType(tn);
         if (!type) return null;
@@ -1940,13 +1958,20 @@ export class Parser extends DiagnosticEmitter {
         initializer = this.parseExpression(tn);
         if (!initializer) return null;
       }
+      let range = tn.range(startPos, tn.pos);
+      if ((flags & CommonFlags.DEFINITE_ASSIGNMENT) && ((flags & CommonFlags.STATIC) || isInterface || initializer)) {
+        this.error(
+          DiagnosticCode.A_definite_assignment_assertion_is_not_permitted_in_this_context,
+          range
+        );
+      }
       let retField = Node.createFieldDeclaration(
         name,
         type,
         initializer,
         decorators,
         flags,
-        tn.range(startPos, tn.pos)
+        range
       );
       tn.skip(Token.SEMICOLON);
       return retField;
diff --git a/tests/parser/definite-assignment-assertion.ts b/tests/parser/definite-assignment-assertion.ts
new file mode 100644
index 00000000..676f66f2
--- /dev/null
+++ b/tests/parser/definite-assignment-assertion.ts
@@ -0,0 +1,9 @@
+class C {
+  x!: i32;
+  x!: i32 = 0;
+  static x!: i32;
+}
+function f(): void {
+  let x!: i32;
+  let x!: i32 = 0;
+}
diff --git a/tests/parser/definite-assignment-assertion.ts.fixture.ts b/tests/parser/definite-assignment-assertion.ts.fixture.ts
new file mode 100644
index 00000000..e90f5786
--- /dev/null
+++ b/tests/parser/definite-assignment-assertion.ts.fixture.ts
@@ -0,0 +1,12 @@
+class C {
+  x!: i32;
+  x!: i32 = 0;
+  static x!: i32;
+}
+function f(): void {
+  let x!: i32;
+  let x!: i32 = 0;
+}
+// ERROR 1255: "A definite assignment assertion '!' is not permitted in this context." in definite-assignment-assertion.ts:3:10
+// ERROR 1255: "A definite assignment assertion '!' is not permitted in this context." in definite-assignment-assertion.ts:4:14
+// ERROR 1255: "A definite assignment assertion '!' is not permitted in this context." in definite-assignment-assertion.ts:8:6
diff --git a/tests/parser/optional-property.ts b/tests/parser/optional-property.ts
new file mode 100644
index 00000000..899c00c6
--- /dev/null
+++ b/tests/parser/optional-property.ts
@@ -0,0 +1,3 @@
+class C {
+  x?: i32;
+}
diff --git a/tests/parser/optional-property.ts.fixture.ts b/tests/parser/optional-property.ts.fixture.ts
new file mode 100644
index 00000000..156a4d03
--- /dev/null
+++ b/tests/parser/optional-property.ts.fixture.ts
@@ -0,0 +1,4 @@
+class C {
+  x: i32;
+}
+// ERROR 219: "Optional properties are not supported." in optional-property.ts:2:9