diff --git a/lib/actions/term-groups.ts b/lib/actions/term-groups.ts
index 154d07a4..d2a83d4e 100644
--- a/lib/actions/term-groups.ts
+++ b/lib/actions/term-groups.ts
@@ -11,7 +11,6 @@ import findBySession from '../utils/term-groups';
 import {getRootGroups} from '../selectors';
 import {setActiveSession, ptyExitSession, userExitSession} from './sessions';
 import {ITermState, ITermGroup, HyperState, HyperDispatch, HyperActions} from '../hyper';
-import {Immutable} from 'seamless-immutable';
 
 function requestSplit(direction: 'VERTICAL' | 'HORIZONTAL') {
   return (activeUid: string) => (dispatch: HyperDispatch, getState: () => HyperState): void => {
@@ -67,7 +66,7 @@ export function setActiveGroup(uid: string) {
 // When we've found the next group which we want to
 // set as active (after closing something), we also need
 // to find the first child group which has a sessionUid.
-const findFirstSession = (state: Immutable<ITermState>, group: Immutable<ITermGroup>): string | undefined => {
+const findFirstSession = (state: ITermState, group: ITermGroup): string | undefined => {
   if (group.sessionUid) {
     return group.sessionUid;
   }
@@ -90,7 +89,7 @@ const findPrevious = <T>(list: T[], old: T) => {
   return index ? list[index - 1] : list[1];
 };
 
-const findNextSessionUid = (state: Immutable<ITermState>, group: Immutable<ITermGroup>) => {
+const findNextSessionUid = (state: ITermState, group: ITermGroup) => {
   // If we're closing a root group (i.e. a whole tab),
   // the next group needs to be a root group as well:
   if (state.activeRootGroup === group.uid) {
diff --git a/lib/actions/ui.ts b/lib/actions/ui.ts
index 53907ccf..af947222 100644
--- a/lib/actions/ui.ts
+++ b/lib/actions/ui.ts
@@ -28,7 +28,7 @@ import {
 
 import {setActiveGroup} from './term-groups';
 import parseUrl from 'parse-url';
-import {HyperState, HyperDispatch, HyperActions} from '../hyper';
+import {HyperState, HyperDispatch, HyperActions, ITermGroups} from '../hyper';
 import {stat, Stats} from 'fs';
 
 export function openContextMenu(uid: string, selection: any) {
@@ -110,7 +110,7 @@ export function windowGeometryUpdated(): HyperActions {
 
 // Find all sessions that are below the given
 // termGroup uid in the hierarchy:
-const findChildSessions = (termGroups: HyperState['termGroups']['termGroups'], uid: string): string[] => {
+const findChildSessions = (termGroups: ITermGroups, uid: string): string[] => {
   const group = termGroups[uid];
   if (group.sessionUid) {
     return [uid];
diff --git a/lib/config.d.ts b/lib/config.d.ts
index 807b463e..9e451a0c 100644
--- a/lib/config.d.ts
+++ b/lib/config.d.ts
@@ -1,5 +1,24 @@
 import {FontWeight} from 'xterm';
 
+export type ColorMap = {
+  black: string;
+  blue: string;
+  cyan: string;
+  green: string;
+  lightBlack: string;
+  lightBlue: string;
+  lightCyan: string;
+  lightGreen: string;
+  lightMagenta: string;
+  lightRed: string;
+  lightWhite: string;
+  lightYellow: string;
+  magenta: string;
+  red: string;
+  white: string;
+  yellow: string;
+};
+
 export type configOptions = {
   autoUpdatePlugins: boolean | string;
   backgroundColor: string;
@@ -7,24 +26,7 @@ export type configOptions = {
   bellSound: string | null;
   bellSoundURL: string | null;
   borderColor: string;
-  colors: {
-    black: string;
-    blue: string;
-    cyan: string;
-    green: string;
-    lightBlack: string;
-    lightBlue: string;
-    lightCyan: string;
-    lightGreen: string;
-    lightMagenta: string;
-    lightRed: string;
-    lightWhite: string;
-    lightYellow: string;
-    magenta: string;
-    red: string;
-    white: string;
-    yellow: string;
-  };
+  colors: ColorMap;
   copyOnSelect: boolean;
   css: string;
   cursorAccentColor: string;
diff --git a/lib/hyper.d.ts b/lib/hyper.d.ts
index 8914f372..97e3eca7 100644
--- a/lib/hyper.d.ts
+++ b/lib/hyper.d.ts
@@ -9,27 +9,28 @@ declare global {
   }
 }
 
-export type ITermGroup = {
+export type ITermGroup = Immutable<{
   uid: string;
   sessionUid: string | null;
   parentUid: string | null;
   direction: 'HORIZONTAL' | 'VERTICAL' | null;
   sizes: number[] | null;
   children: string[];
-};
+}>;
 
-export type ITermGroups = Record<string, ITermGroup>;
+export type ITermGroups = Immutable<Record<string, ITermGroup>>;
 
-export type ITermState = {
-  termGroups: ITermGroups;
+export type ITermState = Immutable<{
+  termGroups: Mutable<ITermGroups>;
   activeSessions: Record<string, string>;
   activeRootGroup: string | null;
-};
+}>;
 
 export type cursorShapes = 'BEAM' | 'UNDERLINE' | 'BLOCK';
 import {FontWeight, Terminal} from 'xterm';
+import {ColorMap} from './config';
 
-export type uiState = {
+export type uiState = Immutable<{
   _lastUpdate: number | null;
   activeUid: string | null;
   activityMarkers: Record<string, boolean>;
@@ -38,24 +39,7 @@ export type uiState = {
   bellSoundURL: string | null;
   bellSound: string | null;
   borderColor: string;
-  colors: {
-    black: string;
-    blue: string;
-    cyan: string;
-    green: string;
-    lightBlack: string;
-    lightBlue: string;
-    lightCyan: string;
-    lightGreen: string;
-    lightMagenta: string;
-    lightRed: string;
-    lightWhite: string;
-    lightYellow: string;
-    magenta: string;
-    red: string;
-    white: string;
-    yellow: string;
-  };
+  colors: ColorMap;
   cols: number | null;
   copyOnSelect: boolean;
   css: string;
@@ -107,7 +91,7 @@ export type uiState = {
   updateVersion: string | null;
   webGLRenderer: boolean;
   webLinksActivationKey: string;
-};
+}>;
 
 export type session = {
   cleared: boolean;
@@ -123,22 +107,19 @@ export type session = {
   splitDirection?: 'HORIZONTAL' | 'VERTICAL';
   activeUid?: string;
 };
-export type sessionState = {
+export type sessionState = Immutable<{
   sessions: Record<string, session>;
   activeUid: string | null;
   write?: any;
-};
+}>;
 
-export {ITermGroupReducer} from './reducers/term-groups';
-import {ITermGroupReducer} from './reducers/term-groups';
+export type ITermGroupReducer = Reducer<ITermState, HyperActions>;
 
-export {IUiReducer} from './reducers/ui';
-import {IUiReducer} from './reducers/ui';
+export type IUiReducer = Reducer<uiState, HyperActions>;
 
-export {ISessionReducer} from './reducers/sessions';
-import {ISessionReducer} from './reducers/sessions';
+export type ISessionReducer = Reducer<sessionState, HyperActions>;
 
-import {Middleware} from 'redux';
+import {Middleware, Reducer} from 'redux';
 export type hyperPlugin = {
   getTabProps: any;
   getTabsProps: any;
@@ -163,9 +144,9 @@ export type hyperPlugin = {
 };
 
 export type HyperState = {
-  ui: Immutable<uiState>;
-  sessions: Immutable<sessionState>;
-  termGroups: Immutable<ITermState>;
+  ui: uiState;
+  sessions: sessionState;
+  termGroups: ITermState;
 };
 
 import {UIActions} from './constants/ui';
@@ -188,8 +169,6 @@ export type HyperActions = (
   | TabActions
 ) & {effect?: () => void};
 
-type immutableRecord<T> = {[k in keyof T]: Immutable<T[k]>};
-
 import configureStore from './store/configure-store';
 export type HyperDispatch = ReturnType<typeof configureStore>['dispatch'];
 
@@ -276,7 +255,7 @@ export type TermGroupOwnProps = {
   fontSmoothing?: string;
   parentProps: TermsProps;
   ref_: (uid: string, term: Term | null) => void;
-  termGroup: Immutable<ITermGroup>;
+  termGroup: ITermGroup;
   terms: Record<string, Term | null>;
 } & Pick<
   TermsProps,
@@ -336,7 +315,7 @@ export type TermProps = {
   bellSoundURL: string | null;
   borderColor: string;
   cleared: boolean;
-  colors: uiState['colors'];
+  colors: ColorMap;
   cols: number | null;
   copyOnSelect: boolean;
   cursorAccentColor?: string;
@@ -379,4 +358,10 @@ export type TermProps = {
   ref_: (uid: string, term: Term | null) => void;
 } & extensionProps;
 
+// Utility types
+
+export type Mutable<T> = T extends Immutable<infer U> ? (Exclude<U, T> extends never ? U : Exclude<U, T>) : T;
+
+export type immutableRecord<T> = {[k in keyof T]: Immutable<T[k]>};
+
 export type Assignable<T, U> = {[k in keyof U]: k extends keyof T ? T[k] : U[k]} & Partial<T>;
diff --git a/lib/reducers/sessions.ts b/lib/reducers/sessions.ts
index 5054bc7f..1850df2f 100644
--- a/lib/reducers/sessions.ts
+++ b/lib/reducers/sessions.ts
@@ -1,4 +1,4 @@
-import Immutable, {Immutable as ImmutableType} from 'seamless-immutable';
+import Immutable from 'seamless-immutable';
 import {decorateSessionsReducer} from '../utils/plugins';
 import {
   SESSION_ADD,
@@ -13,10 +13,10 @@ import {
   SESSION_SEARCH,
   SESSION_SEARCH_CLOSE
 } from '../constants/sessions';
-import {sessionState, session, HyperActions} from '../hyper';
+import {sessionState, session, Mutable, ISessionReducer} from '../hyper';
 
-const initialState: ImmutableType<sessionState> = Immutable({
-  sessions: {} as Record<string, session>,
+const initialState: sessionState = Immutable<Mutable<sessionState>>({
+  sessions: {},
   activeUid: null
 });
 
@@ -35,7 +35,7 @@ function Session(obj: Immutable.DeepPartial<session>) {
   return Immutable(x).merge(obj);
 }
 
-function deleteSession(state: ImmutableType<sessionState>, uid: string) {
+function deleteSession(state: sessionState, uid: string) {
   return state.updateIn(['sessions'], (sessions: typeof state['sessions']) => {
     const sessions_ = sessions.asMutable();
     delete sessions_[uid];
@@ -43,7 +43,7 @@ function deleteSession(state: ImmutableType<sessionState>, uid: string) {
   });
 }
 
-const reducer = (state: ImmutableType<sessionState> = initialState, action: HyperActions) => {
+const reducer: ISessionReducer = (state = initialState, action) => {
   switch (action.type) {
     case SESSION_ADD:
       return state.set('activeUid', action.uid).setIn(
@@ -134,6 +134,4 @@ const reducer = (state: ImmutableType<sessionState> = initialState, action: Hype
   }
 };
 
-export type ISessionReducer = typeof reducer;
-
 export default decorateSessionsReducer(reducer);
diff --git a/lib/reducers/term-groups.ts b/lib/reducers/term-groups.ts
index e4e24169..3880f7ab 100644
--- a/lib/reducers/term-groups.ts
+++ b/lib/reducers/term-groups.ts
@@ -4,17 +4,17 @@ import {TERM_GROUP_EXIT, TERM_GROUP_RESIZE} from '../constants/term-groups';
 import {SESSION_ADD, SESSION_SET_ACTIVE, SessionAddAction} from '../constants/sessions';
 import findBySession from '../utils/term-groups';
 import {decorateTermGroupsReducer} from '../utils/plugins';
-import {ITermGroup, ITermState, ITermGroups, HyperActions} from '../hyper';
+import {ITermGroup, ITermState, ITermGroups, ITermGroupReducer, Mutable} from '../hyper';
 
 const MIN_SIZE = 0.05;
-const initialState = Immutable<ITermState>({
+const initialState: ITermState = Immutable<Mutable<ITermState>>({
   termGroups: {},
   activeSessions: {},
   activeRootGroup: null
 });
 
-function TermGroup(obj: Immutable.DeepPartial<ITermGroup>) {
-  const x: ITermGroup = {
+function TermGroup(obj: Immutable.DeepPartial<Mutable<ITermGroup>>) {
+  const x: Mutable<ITermGroup> = {
     uid: '',
     sessionUid: null,
     parentUid: null,
@@ -26,7 +26,7 @@ function TermGroup(obj: Immutable.DeepPartial<ITermGroup>) {
 }
 
 // Recurse upwards until we find a root term group (no parent).
-const findRootGroup = (termGroups: ImmutableType<ITermGroups>, uid: string): ImmutableType<ITermGroup> => {
+const findRootGroup = (termGroups: ITermGroups, uid: string): ITermGroup => {
   const current = termGroups[uid];
   if (!current.parentUid) {
     return current;
@@ -35,7 +35,7 @@ const findRootGroup = (termGroups: ImmutableType<ITermGroups>, uid: string): Imm
   return findRootGroup(termGroups, current.parentUid);
 };
 
-const setActiveGroup = (state: ImmutableType<ITermState>, action: {uid: string}) => {
+const setActiveGroup = (state: ITermState, action: {uid: string}) => {
   if (!action.uid) {
     return state.set('activeRootGroup', null);
   }
@@ -66,7 +66,7 @@ const removalRebalance = (oldSizes: ImmutableType<number[]>, index: number) => {
   );
 };
 
-const splitGroup = (state: ImmutableType<ITermState>, action: SessionAddAction) => {
+const splitGroup = (state: ITermState, action: SessionAddAction) => {
   const {splitDirection, uid, activeUid} = action;
   const activeGroup = findBySession(state, activeUid!)!;
   // If we're splitting in the same direction as the current active
@@ -132,11 +132,7 @@ const splitGroup = (state: ImmutableType<ITermState>, action: SessionAddAction)
 // Replace the parent by the given child in the tree,
 // used when we remove another child and we're left
 // with a one-to-one mapping between parent and child.
-const replaceParent = (
-  state: ImmutableType<ITermState>,
-  parent: ImmutableType<ITermGroup>,
-  child: ImmutableType<ITermGroup>
-) => {
+const replaceParent = (state: ITermState, parent: ITermGroup, child: ITermGroup) => {
   if (parent.parentUid) {
     const parentParent = state.termGroups[parent.parentUid];
     // If the parent we're replacing has a parent,
@@ -161,7 +157,7 @@ const replaceParent = (
     .setIn(['termGroups', child.uid, 'parentUid'], parent.parentUid);
 };
 
-const removeGroup = (state: ImmutableType<ITermState>, uid: string) => {
+const removeGroup = (state: ITermState, uid: string) => {
   const group = state.termGroups[uid];
   // when close tab with multiple panes, it remove group from parent to child. so maybe the parentUid exists but parent group have removed.
   // it's safe to remove the group.
@@ -188,7 +184,7 @@ const removeGroup = (state: ImmutableType<ITermState>, uid: string) => {
     .set('activeSessions', state.activeSessions.without(uid));
 };
 
-const resizeGroup = (state: ImmutableType<ITermState>, uid: string, sizes: number[]) => {
+const resizeGroup = (state: ITermState, uid: string, sizes: number[]) => {
   // Make sure none of the sizes fall below MIN_SIZE:
   if (sizes.find((size) => size < MIN_SIZE)) {
     return state;
@@ -197,7 +193,7 @@ const resizeGroup = (state: ImmutableType<ITermState>, uid: string, sizes: numbe
   return state.setIn(['termGroups', uid, 'sizes'], sizes);
 };
 
-const reducer = (state = initialState, action: HyperActions) => {
+const reducer: ITermGroupReducer = (state = initialState, action) => {
   switch (action.type) {
     case SESSION_ADD: {
       if (action.splitDirection) {
@@ -227,6 +223,4 @@ const reducer = (state = initialState, action: HyperActions) => {
   }
 };
 
-export type ITermGroupReducer = typeof reducer;
-
 export default decorateTermGroupsReducer(reducer);
diff --git a/lib/reducers/ui.ts b/lib/reducers/ui.ts
index 26e7f1b2..4405f6fc 100644
--- a/lib/reducers/ui.ts
+++ b/lib/reducers/ui.ts
@@ -24,7 +24,7 @@ import {
   SESSION_SET_CWD
 } from '../constants/sessions';
 import {UPDATE_AVAILABLE} from '../constants/updater';
-import {uiState, HyperActions} from '../hyper';
+import {uiState, Mutable, IUiReducer} from '../hyper';
 
 const allowedCursorShapes = new Set(['BEAM', 'BLOCK', 'UNDERLINE']);
 const allowedCursorBlinkValues = new Set([true, false]);
@@ -33,7 +33,7 @@ const allowedHamburgerMenuValues = new Set([true, false]);
 const allowedWindowControlsValues = new Set([true, false, 'left']);
 
 // Populate `config-default.js` from this :)
-const initial: ImmutableType<uiState> = Immutable({
+const initial: uiState = Immutable<Mutable<uiState>>({
   cols: null,
   rows: null,
   scrollback: 1000,
@@ -57,7 +57,7 @@ const initial: ImmutableType<uiState> = Immutable({
   letterSpacing: 0,
   css: '',
   termCSS: '',
-  openAt: {} as Record<string, number>,
+  openAt: {},
   resizeAt: 0,
   colors: {
     black: '#000000',
@@ -77,7 +77,7 @@ const initial: ImmutableType<uiState> = Immutable({
     lightCyan: '#68FDFE',
     lightWhite: '#FFFFFF'
   },
-  activityMarkers: {} as Record<string, boolean>,
+  activityMarkers: {},
   notifications: {
     font: false,
     resize: false,
@@ -115,7 +115,7 @@ const initial: ImmutableType<uiState> = Immutable({
 
 const currentWindow = remote.getCurrentWindow();
 
-const reducer = (state = initial, action: HyperActions) => {
+const reducer: IUiReducer = (state = initial, action) => {
   let state_ = state;
   let isMax;
   switch (action.type) {
@@ -127,7 +127,7 @@ const reducer = (state = initial, action: HyperActions) => {
         // font size changed from the config
         .merge(
           (() => {
-            const ret: Immutable.DeepPartial<uiState> = {};
+            const ret: Immutable.DeepPartial<Mutable<uiState>> = {};
 
             if (config.scrollback) {
               ret.scrollback = config.scrollback;
@@ -452,6 +452,4 @@ const reducer = (state = initial, action: HyperActions) => {
   return state_;
 };
 
-export type IUiReducer = typeof reducer;
-
 export default decorateUIReducer(reducer);
diff --git a/lib/utils/term-groups.ts b/lib/utils/term-groups.ts
index b59731a3..9e3706e1 100644
--- a/lib/utils/term-groups.ts
+++ b/lib/utils/term-groups.ts
@@ -1,7 +1,6 @@
 import {ITermState} from '../hyper';
-import {Immutable} from 'seamless-immutable';
 
-export default function findBySession(termGroupState: Immutable<ITermState>, sessionUid: string) {
+export default function findBySession(termGroupState: ITermState, sessionUid: string) {
   const {termGroups} = termGroupState;
   return Object.keys(termGroups)
     .map((uid) => termGroups[uid])