diff --git a/types/seamless-immutable/index.d.ts b/types/seamless-immutable/index.d.ts
index 31c6eea8be..853b8e2113 100644
--- a/types/seamless-immutable/index.d.ts
+++ b/types/seamless-immutable/index.d.ts
@@ -32,8 +32,8 @@ declare namespace SeamlessImmutable {
         prototype?: any;
     }
 
-    interface AsMutableOptions {
-        deep: boolean;
+    interface AsMutableOptions<TDeep extends boolean = boolean> {
+        deep: TDeep;
     }
 
     interface ImmutableObjectMixin<T> {
@@ -63,7 +63,9 @@ declare namespace SeamlessImmutable {
         getIn(propertyPath: string[]): Immutable<any>;
         getIn<TValue>(propertyPath: string[], defaultValue: TValue): Immutable<TValue>;
 
-        asMutable(opts?: AsMutableOptions): T;
+        asMutable(opts?: AsMutableOptions<false>): { [K in keyof T]: Immutable<T[K]> };
+        asMutable(opts: AsMutableOptions<true>): T;
+        asMutable(opts: AsMutableOptions): T | { [K in keyof T]: Immutable<T[K]> };
 
         merge(part: DeepPartial<T>, config?: MergeConfig): Immutable<T>;
 
@@ -91,29 +93,38 @@ declare namespace SeamlessImmutable {
     type ImmutableObject<T> = ImmutableObjectMixin<T> & { readonly [P in keyof T]: Immutable<T[P]> };
 
     /** An ImmutableArray provides read-only access to the array elements, and provides functions (such as `map()`) that return immutable data structures. */
-    type ImmutableArray<T> = Readonly<ImmutableArray.Remaining<T> & ImmutableArray.Additions<T> & ImmutableArray.Overrides<T>>;
+    type ImmutableArray<T> = Readonly<ImmutableArray.Remaining<T>> & ImmutableArray.Additions<T> & ImmutableArray.Overrides<T> & ImmutableArray.ReadOnlyIndexer<T>;
     namespace ImmutableArray {
         /** New methods added by seamless-immutable. */
         interface Additions<T> {
-            asMutable(opts?: AsMutableOptions): T[];
+            asMutable(opts?: AsMutableOptions<false>): Array<Immutable<T>>;
+            asMutable(opts: AsMutableOptions<true>): T[];
+            asMutable(opts: AsMutableOptions): T[] | Array<Immutable<T>>;
+
             asObject<U extends object = {}, K extends keyof U = keyof U>(toKeyValue: (item: T) => [K, U[K]]): Immutable<U>;
             flatMap<TTarget>(mapFunction: (item: T) => TTarget): Immutable<TTarget extends any[] ? TTarget : TTarget[]>;
         }
 
         /** Custom implementation of the array functions, which return Immutable. */
         interface Overrides<T> {
-            map<TTarget>(mapFuction: (item: T) => TTarget): Immutable<TTarget[]>;
-            filter(filterFunction: (item: T) => boolean): Immutable<T[]>;
+            forEach(callbackfn: (value: Immutable<T>, index: number, array: Immutable<T[]>) => void, thisArg?: any): void;
+            map<TTarget>(mapFuction: (item: Immutable<T>) => TTarget): Immutable<TTarget[]>;
+            filter(filterFunction: (item: Immutable<T>) => boolean): Immutable<T[]>;
             slice(start?: number, end?: number): Immutable<T[]>;
-            concat(...arr: Array<T|T[]|Immutable<T|T[]>>): Immutable<T[]>;
-            reduce(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): Immutable<T>;
-            reduce<TTarget>(callbackfn: (previousValue: TTarget, currentValue: T, currentIndex: number, array: T[]) => TTarget, initialValue?: TTarget): Immutable<TTarget>;
-            reduceRight(callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: T[]) => T): Immutable<T>;
-            reduceRight<TTarget>(callbackfn: (previousValue: TTarget, currentValue: T, currentIndex: number, array: T[]) => TTarget, initialValue?: TTarget): Immutable<TTarget>;
+            concat(...arr: Array<T|T[]|Immutable<T>|Array<Immutable<T>>|Immutable<T[]>>): Immutable<T[]>;
+            reduce(callbackfn: (previousValue: Immutable<T>, currentValue: Immutable<T>, currentIndex: number, array: Immutable<T[]>) => T): Immutable<T>;
+            reduce<TTarget>(callbackfn: (previousValue: TTarget, currentValue: Immutable<T>, currentIndex: number, array: Immutable<T[]>) => TTarget, initialValue?: TTarget): Immutable<TTarget>;
+            reduceRight(callbackfn: (previousValue: Immutable<T>, currentValue: Immutable<T>, currentIndex: number, array: Immutable<T[]>) => T): Immutable<T>;
+            reduceRight<TTarget>(callbackfn: (previousValue: TTarget, currentValue: Immutable<T>, currentIndex: number, array: Immutable<T[]>) => TTarget, initialValue?: TTarget): Immutable<TTarget>;
+        }
+
+        /** Merging this into Overrides breaks stuff, so this is split out */
+        interface ReadOnlyIndexer<T> {
+            readonly [key: number]: Immutable<T>;
         }
 
         /** These methods are banned by seamless-immutable. */
-        type MutatingArrayMethods = Extract<keyof any[], 'push' | 'pop' | 'sort' | 'splice' | 'shift' | 'unshift' | 'reverse'>;
+        type MutatingArrayMethods = Extract<keyof any[], 'push' | 'pop' | 'sort' | 'splice' | 'shift' | 'unshift' | 'reverse' | number>;
 
         /** NOTE: These methods mutate data, but seamless-immutable does not ban them. We will ban them in our type definitions. */
         type AdditionalMutatingArrayMethods = Extract<keyof any[], 'copyWithin' | 'fill'>;
@@ -160,7 +171,7 @@ declare namespace SeamlessImmutable {
 
     function from<T>(obj: T, options?: Options): Immutable<T>;
 
-    function isImmutable(target: any): boolean;
+    function isImmutable<T>(target: T | Immutable<T>): target is Immutable<T>;
     function ImmutableError(message: string): Error;
 
     function replace<T, S>(obj: Immutable<T>, valueObj: S, options?: ReplaceConfig): Immutable<S>;
diff --git a/types/seamless-immutable/seamless-immutable-tests.ts b/types/seamless-immutable/seamless-immutable-tests.ts
index 773a7b9320..e2982b9e08 100644
--- a/types/seamless-immutable/seamless-immutable-tests.ts
+++ b/types/seamless-immutable/seamless-immutable-tests.ts
@@ -15,6 +15,13 @@ interface ExtendedUser extends User {
     address: Address;
 }
 
+// A result type for asMutable({ deep: false } | undefined)
+interface NonDeepMutableExtendedUser {
+    firstName: string;
+    lastName: string;
+    address: Immutable.Immutable<Address>;
+}
+
 //
 // Constructors
 // ---------------------------------------------------------------
@@ -89,6 +96,27 @@ interface ExtendedUser extends User {
 {
     const array: Immutable.Immutable<User[]> = Immutable.from([ { firstName: 'Angry', lastName: 'Monkey' } ]);
 
+    const immutableElement: Immutable.Immutable<User> = array[0];
+    // $ExpectError
+    immutableElement.firstName = 'Krusty';
+
+    const asMutableDefault: Array<Immutable.Immutable<User>> = array.asMutable();
+    // Can't mutate beyond 1 level deep (in this case only the array itself is mutable, not the items)
+    // $ExpectError
+    asMutableDefault[0].firstName = 'Krusty';
+
+    const asMutableNotDeep: Array<Immutable.Immutable<User>> = array.asMutable({ deep: false });
+    // Can't mutate beyond 1 level deep (in this case only the array itself is mutable, not the items)
+    // $ExpectError
+    asMutableNotDeep[0].firstName = 'Krusty';
+
+    // Can mutate at any depth
+    const asMutableDeep: User[] = array.asMutable({ deep: true });
+    asMutableDeep[0].firstName = 'Krusty';
+
+    const opaqueMutableParam = { deep: true };
+    const asMutableOpaque: Array<User | Immutable.Immutable<User>> = array.asMutable(opaqueMutableParam);
+
     // keys. Call the mutable array's 'keys' to ensure compatability
     const mutableKeys = array.asMutable().keys();
     const keys: typeof mutableKeys = array.keys();
@@ -119,7 +147,7 @@ interface ExtendedUser extends User {
     slice4.asMutable();
 
     // concat. Call the mutable array's 'concat' with the same args to ensure compatability. Make sure the output array is immutable.
-    array.asMutable().concat({ firstName: 'Happy', lastName: 'Cat' });
+    array.asMutable().concat(Immutable({ firstName: 'Happy', lastName: 'Cat' }));
     const concat: Immutable.Immutable<User[]> = array
         .concat({ firstName: 'Happy', lastName: 'Cat' }, { firstName: 'Silly', lastName: 'Cat' })
         .concat([{firstName: 'saucy', lastName: 'Cat'}], {firstName: 'Fussy', lastName: 'Cat'})
@@ -204,8 +232,25 @@ interface ExtendedUser extends User {
     const updatedUser12: Immutable.Immutable<ExtendedUser> = immutableUserEx.setIn([ data.propertyId, 'line1' ], 'Small house');
 
     // asMutable
-    const mutableUser21: User = immutableUser.asMutable();
-    const mutableUser22: User = immutableUser.asMutable({ deep: true });
+    // Can't mutate beyond 1 level deep
+    const mutableUser21 = immutableUserEx.asMutable();
+    mutableUser21.firstName = 'Krusty';
+    // $ExpectError
+    mutableUser21.address.line1 = 'Example';
+
+    // Can't mutate beyond 1 level deep
+    const mutableUser22 = immutableUserEx.asMutable({ deep: false });
+    mutableUser22.firstName = 'Krusty';
+    // $ExpectError
+    mutableUser22.address.line1 = 'Example';
+
+    // Can mutate at any depth
+    const mutableUser23: ExtendedUser = immutableUserEx.asMutable({ deep: true });
+    mutableUser23.firstName = 'Krusty';
+    mutableUser23.address.line1 = 'Example';
+
+    const opaqueMutableParam = { deep: true }; // treats as { deep: boolean }
+    const mutableUser24: ExtendedUser | NonDeepMutableExtendedUser = immutableUserEx.asMutable(opaqueMutableParam);
 
     // merge: merged part is strongly checked as a deeply partial object
     const mergedUser: Immutable.Immutable<User> = immutableUserEx.merge({ address: { line1: 'Small house' }, firstName: 'Jack' });