diff --git a/js/constants.js b/js/constants.ts
similarity index 78%
rename from js/constants.js
rename to js/constants.ts
index ff2ffcbb..fa784b72 100644
--- a/js/constants.js
+++ b/js/constants.ts
@@ -1,5 +1,17 @@
+import { Band, MediaTagRequestStatus } from "./types";
 import * as baseSkin from "./baseSkin.json";
-export const BANDS = [60, 170, 310, 600, 1000, 3000, 6000, 12000, 14000, 16000];
+export const BANDS: Band[] = [
+  60,
+  170,
+  310,
+  600,
+  1000,
+  3000,
+  6000,
+  12000,
+  14000,
+  16000
+];
 
 export const WINDOWS = {
   MAIN: "main",
@@ -12,7 +24,11 @@ export const LOAD_STYLE = {
   PLAY: "PLAY"
 };
 
-export const MEDIA_TAG_REQUEST_STATUS = {
+// TODO: Make this an enum?
+export const MEDIA_TAG_REQUEST_STATUS: Record<
+  MediaTagRequestStatus,
+  MediaTagRequestStatus
+> = {
   INITIALIZED: "INITIALIZED",
   FAILED: "FAILED",
   COMPLETE: "COMPLETE",
diff --git a/js/reducers/equalizer.js b/js/reducers/equalizer.ts
similarity index 60%
rename from js/reducers/equalizer.js
rename to js/reducers/equalizer.ts
index 9ab95fbb..172a7482 100644
--- a/js/reducers/equalizer.js
+++ b/js/reducers/equalizer.ts
@@ -1,3 +1,5 @@
+import { EqualizerState, Action } from "./../types";
+
 import { BANDS } from "../constants";
 import {
   SET_BAND_VALUE,
@@ -6,20 +8,28 @@ import {
   SET_EQ_OFF
 } from "../actionTypes";
 
-const equalizer = (state, action) => {
-  if (!state) {
-    state = {
-      on: true,
-      auto: false,
-      sliders: {
-        preamp: 50
-      }
-    };
-    BANDS.forEach(band => {
-      state.sliders[band] = 50;
-    });
-    return state;
+const defaultState = {
+  on: true,
+  auto: false,
+  sliders: {
+    preamp: 50,
+    60: 50,
+    170: 50,
+    310: 50,
+    600: 50,
+    1000: 50,
+    3000: 50,
+    6000: 50,
+    12000: 50,
+    14000: 50,
+    16000: 50
   }
+};
+
+const equalizer = (
+  state: EqualizerState = defaultState,
+  action: Action
+): EqualizerState => {
   switch (action.type) {
     case SET_BAND_VALUE:
       const newSliders = { ...state.sliders, [action.band]: action.value };
diff --git a/js/reducers/index.js b/js/reducers/index.ts
similarity index 81%
rename from js/reducers/index.js
rename to js/reducers/index.ts
index d871053a..28c6c27f 100644
--- a/js/reducers/index.js
+++ b/js/reducers/index.ts
@@ -1,3 +1,4 @@
+import { AppState, Action } from "../types";
 import { combineReducers } from "redux";
 
 import playlist from "./playlist";
@@ -9,7 +10,7 @@ import equalizer from "./equalizer";
 import network from "./network";
 import settings from "./settings";
 
-const reducer = combineReducers({
+const reducer = combineReducers<AppState, Action>({
   userInput,
   windows,
   display,
diff --git a/js/reducers/media.ts b/js/reducers/media.ts
index 5a3cd9db..8ff868f3 100644
--- a/js/reducers/media.ts
+++ b/js/reducers/media.ts
@@ -17,26 +17,28 @@ import {
 } from "../actionTypes";
 import { TIME_MODE, MEDIA_STATUS } from "../constants";
 
-const media = (state: MediaState, action: Action): MediaState => {
-  if (!state) {
-    return {
-      timeMode: TIME_MODE.ELAPSED,
-      timeElapsed: 0,
-      length: null, // Consider renaming to "duration"
-      kbps: null,
-      khz: null,
-      // The winamp ini file declares the default volume as "200".
-      // The UI seems to show a default volume near 78, which would
-      // math with the default value being 200 out of 255.
-      volume: Math.round((200 / 255) * 100),
-      balance: 0,
-      channels: null,
-      shuffle: false,
-      repeat: false,
-      // TODO: Enforce possible values
-      status: MEDIA_STATUS.STOPPED
-    };
-  }
+const defaultState = {
+  timeMode: TIME_MODE.ELAPSED,
+  timeElapsed: 0,
+  length: null, // Consider renaming to "duration"
+  kbps: null,
+  khz: null,
+  // The winamp ini file declares the default volume as "200".
+  // The UI seems to show a default volume near 78, which would
+  // math with the default value being 200 out of 255.
+  volume: Math.round((200 / 255) * 100),
+  balance: 0,
+  channels: null,
+  shuffle: false,
+  repeat: false,
+  // TODO: Enforce possible values
+  status: MEDIA_STATUS.STOPPED
+};
+
+const media = (
+  state: MediaState = defaultState,
+  action: Action
+): MediaState => {
   switch (action.type) {
     // TODO: Make these constants
     case PLAY:
@@ -53,7 +55,7 @@ const media = (state: MediaState, action: Action): MediaState => {
       const newMode =
         state.timeMode === TIME_MODE.REMAINING
           ? TIME_MODE.ELAPSED
-          : TIME_MODE.TIME_REMAINING;
+          : TIME_MODE.REMAINING;
       return { ...state, timeMode: newMode };
     case UPDATE_TIME_ELAPSED:
       return { ...state, timeElapsed: action.elapsed };
diff --git a/js/reducers/playlist.js b/js/reducers/playlist.ts
similarity index 78%
rename from js/reducers/playlist.js
rename to js/reducers/playlist.ts
index c89e0c5e..2e598469 100644
--- a/js/reducers/playlist.js
+++ b/js/reducers/playlist.ts
@@ -1,3 +1,4 @@
+import { PlaylistState, PlaylistTrack, Action } from "../types";
 import {
   SET_MEDIA,
   CLICKED_TRACK,
@@ -25,6 +26,8 @@ import { MEDIA_TAG_REQUEST_STATUS } from "../constants";
 import { filenameFromUrl } from "../fileUtils";
 import { shuffle, moveSelected, objectMap, objectFilter } from "../utils";
 
+const foo = objectMap({ foo: 1 }, String);
+
 const defaultPlaylistState = {
   trackOrder: [],
   currentTrack: null,
@@ -32,16 +35,22 @@ const defaultPlaylistState = {
   lastSelectedIndex: null
 };
 
-const playlist = (state = defaultPlaylistState, action) => {
+const playlist = (
+  state: PlaylistState = defaultPlaylistState,
+  action: Action
+): PlaylistState => {
   switch (action.type) {
     case CLICKED_TRACK:
       const clickedId = String(state.trackOrder[action.index]);
       return {
         ...state,
-        tracks: objectMap(state.tracks, (track, id) => ({
-          ...track,
-          selected: id === clickedId
-        })),
+        tracks: objectMap<PlaylistTrack, PlaylistTrack>(
+          state.tracks,
+          (track, id) => ({
+            ...track,
+            selected: id === clickedId
+          })
+        ),
         lastSelectedIndex: action.index
       };
     case CTRL_CLICKED_TRACK:
@@ -68,31 +77,43 @@ const playlist = (state = defaultPlaylistState, action) => {
       const selected = new Set(state.trackOrder.slice(start, end + 1));
       return {
         ...state,
-        tracks: objectMap(state.tracks, (track, trackId) => ({
-          ...track,
-          selected: selected.has(Number(trackId))
-        }))
+        tracks: objectMap<PlaylistTrack, PlaylistTrack>(
+          state.tracks,
+          (track, trackId) => ({
+            ...track,
+            selected: selected.has(Number(trackId))
+          })
+        )
       };
     case SELECT_ALL:
       return {
         ...state,
-        tracks: objectMap(state.tracks, track => ({ ...track, selected: true }))
+        tracks: objectMap<PlaylistTrack, PlaylistTrack>(
+          state.tracks,
+          track => ({ ...track, selected: true })
+        )
       };
     case SELECT_ZERO:
       return {
         ...state,
-        tracks: objectMap(state.tracks, track => ({
-          ...track,
-          selected: false
-        }))
+        tracks: objectMap<PlaylistTrack, PlaylistTrack>(
+          state.tracks,
+          track => ({
+            ...track,
+            selected: false
+          })
+        )
       };
     case INVERT_SELECTION:
       return {
         ...state,
-        tracks: objectMap(state.tracks, track => ({
-          ...track,
-          selected: !track.selected
-        }))
+        tracks: objectMap<PlaylistTrack, PlaylistTrack>(
+          state.tracks,
+          track => ({
+            ...track,
+            selected: !track.selected
+          })
+        )
       };
     case REMOVE_ALL_TRACKS:
       // TODO: Consider disposing of ObjectUrls
@@ -112,7 +133,9 @@ const playlist = (state = defaultPlaylistState, action) => {
         trackOrder: state.trackOrder.filter(
           trackId => !actionIds.includes(trackId)
         ),
-        currentTrack: actionIds.includes(currentTrack) ? null : currentTrack,
+        currentTrack: actionIds.includes(Number(currentTrack))
+          ? null
+          : currentTrack,
         tracks: objectFilter(
           state.tracks,
           (track, trackId) => !action.ids.includes(trackId)
@@ -159,18 +182,20 @@ const playlist = (state = defaultPlaylistState, action) => {
         // TODO: This could probably be made to work, but we clear it just to be safe.
         lastSelectedIndex: null
       };
-    case SET_MEDIA:
+    case SET_MEDIA: {
+      const newTrack = {
+        ...state.tracks[action.id],
+        duration: action.length
+      };
       return {
         ...state,
         tracks: {
           ...state.tracks,
-          [action.id]: {
-            ...state.tracks[action.id],
-            duration: action.length
-          }
+          [action.id]: newTrack
         }
       };
-    case SET_MEDIA_TAGS:
+    }
+    case SET_MEDIA_TAGS: {
       return {
         ...state,
         tracks: {
@@ -184,6 +209,7 @@ const playlist = (state = defaultPlaylistState, action) => {
           }
         }
       };
+    }
     case MEDIA_TAG_REQUEST_INITIALIZED:
       return {
         ...state,
@@ -206,17 +232,19 @@ const playlist = (state = defaultPlaylistState, action) => {
           }
         }
       };
-    case SET_MEDIA_DURATION:
+    case SET_MEDIA_DURATION: {
+      const newTrack = {
+        ...state.tracks[action.id],
+        duration: action.duration
+      };
       return {
         ...state,
         tracks: {
           ...state.tracks,
-          [action.id]: {
-            ...state.tracks[action.id],
-            duration: action.duration
-          }
+          [action.id]: newTrack
         }
       };
+    }
     case PLAY_TRACK:
     case BUFFER_TRACK:
       return {
@@ -241,7 +269,10 @@ const playlist = (state = defaultPlaylistState, action) => {
 
 export default playlist;
 
-export const getTrackDisplayName = (state, id) => {
+export const getTrackDisplayName = (
+  state: PlaylistState,
+  id: number
+): string | null => {
   const track = state.tracks[id];
   if (track == null) {
     return null;
diff --git a/js/reducers/windows.js b/js/reducers/windows.ts
similarity index 90%
rename from js/reducers/windows.js
rename to js/reducers/windows.ts
index 6e4cf34d..1cdcf9e6 100644
--- a/js/reducers/windows.js
+++ b/js/reducers/windows.ts
@@ -1,3 +1,4 @@
+import { WindowState, Action } from "../types";
 import { WINDOWS } from "../constants";
 import {
   SET_FOCUSED_WINDOW,
@@ -10,7 +11,7 @@ import {
   TOGGLE_WINDOW_SHADE_MODE
 } from "../actionTypes";
 
-const defaultWindowsState = {
+const defaultWindowsState: WindowState = {
   focused: WINDOWS.MAIN,
   genWindows: {
     // TODO: Remove static capabilites and derive them from ids/generic
@@ -54,7 +55,10 @@ const defaultWindowsState = {
   positions: {}
 };
 
-const windows = (state = defaultWindowsState, action) => {
+const windows = (
+  state: WindowState = defaultWindowsState,
+  action: Action
+): WindowState => {
   switch (action.type) {
     case SET_FOCUSED_WINDOW:
       return { ...state, focused: action.window };
@@ -62,8 +66,9 @@ const windows = (state = defaultWindowsState, action) => {
       const { canShade } = state.genWindows[action.windowId];
       if (!canShade) {
         throw new Error(
-          "Tried to shade/unshade a window that cannot be shaded:",
-          action.windowId
+          `Tried to shade/unshade a window that cannot be shaded: ${
+            action.windowId
+          }`
         );
       }
       return {
@@ -133,8 +138,7 @@ const windows = (state = defaultWindowsState, action) => {
       const { canResize } = state.genWindows[action.windowId];
       if (!canResize) {
         throw new Error(
-          "Tried to resize a window that cannot be resized:",
-          action.windowId
+          `Tried to resize a window that cannot be resized: ${action.windowId}`
         );
       }
       return {
diff --git a/js/types.ts b/js/types.ts
index 136b7246..95e27aad 100644
--- a/js/types.ts
+++ b/js/types.ts
@@ -3,7 +3,18 @@ type Skin = {
   name: string;
 };
 
-type Band = null; // TODO: Use a real type here.
+export type Band =
+  | 60
+  | 170
+  | 310
+  | 600
+  | 1000
+  | 3000
+  | 6000
+  | 12000
+  | 14000
+  | 16000;
+type Slider = Band | "preamp";
 
 // TODO: Use a type to ensure these keys mirror the CURSORS constant in
 // skinParser.js
@@ -20,6 +31,8 @@ type SkinImages = { [sprite: string]: string };
 // TODO: type these keys
 type SkinRegion = { [windowName: string]: string[] };
 
+type WindowId = string;
+
 // TODO: Fill these out once we actually use them.
 type SkinData = {
   skinImages: SkinImages;
@@ -69,9 +82,15 @@ export type Action =
     }
   | {
       type: "ADD_TRACK_FROM_URL";
+      atIndex: number | null;
+      id: string;
+      defaultName: string;
+      duration: number | null;
+      url: string;
     }
   | {
       type: "SET_MEDIA";
+      id: number;
       length: number;
       kbps: number;
       khz: number;
@@ -163,6 +182,127 @@ export type Action =
   | {
       type: "SET_DUMMY_VIZ_DATA";
       data: null;
+    }
+  | {
+      type: "SET_BAND_VALUE";
+      band: Slider;
+      value: number;
+    }
+  | {
+      type: "SET_EQ_ON";
+    }
+  | {
+      type: "SET_EQ_OFF";
+    }
+  | {
+      type: "SET_EQ_AUTO";
+      value: boolean;
+    }
+  | {
+      type: "SET_FOCUSED_WINDOW";
+      window: WindowId;
+    }
+  | {
+      type: "TOGGLE_WINDOW_SHADE_MODE";
+      windowId: WindowId;
+    }
+  | {
+      type: "TOGGLE_WINDOW";
+      windowId: WindowId;
+    }
+  | {
+      type: "CLOSE_WINDOW";
+      windowId: WindowId;
+    }
+  | {
+      type: "SET_WINDOW_VISIBILITY";
+      windowId: WindowId;
+      hidden: boolean;
+    }
+  | {
+      type: "ADD_GEN_WINDOW";
+      windowId: WindowId;
+      title: string;
+      open: boolean;
+    }
+  | {
+      type: "WINDOW_SIZE_CHANGED";
+      windowId: WindowId;
+      size: [number, number];
+    }
+  | {
+      type: "UPDATE_WINDOW_POSITIONS";
+      positions: {};
+    }
+  | {
+      type: "CLICKED_TRACK";
+      index: number;
+    }
+  | {
+      type: "CTRL_CLICKED_TRACK";
+      index: number;
+    }
+  | {
+      type: "SHIFT_CLICKED_TRACK";
+      index: number;
+    }
+  | {
+      type: "SELECT_ALL";
+    }
+  | {
+      type: "SELECT_ZERO";
+    }
+  | {
+      type: "INVERT_SELECTION";
+    }
+  | {
+      type: "REMOVE_ALL_TRACKS";
+    }
+  | {
+      type: "REMOVE_TRACKS";
+      ids: string[];
+    }
+  | {
+      type: "REVERSE_LIST";
+    }
+  | {
+      type: "RANDOMIZE_LIST";
+    }
+  | {
+      type: "SET_TRACK_ORDER";
+      trackOrder: number[];
+    }
+  | {
+      type: "SET_MEDIA_TAGS";
+      id: number;
+      title: string;
+      artist: string;
+      albumArtUrl: string;
+    }
+  | {
+      type: "MEDIA_TAG_REQUEST_INITIALIZED";
+      id: number;
+    }
+  | {
+      type: "MEDIA_TAG_REQUEST_FAILED";
+      id: number;
+    }
+  | {
+      type: "SET_MEDIA_DURATION";
+      id: number;
+      duration: number;
+    }
+  | {
+      type: "PLAY_TRACK";
+      id: number;
+    }
+  | {
+      type: "BUFFER_TRACK";
+      id: number;
+    }
+  | {
+      type: "DRAG_SELECTED";
+      offset: number;
     };
 
 export interface SettingsState {
@@ -189,7 +329,7 @@ export interface MediaState {
 
 export interface UserInputState {
   focus: string | null; // TODO: Convert this to an enum?
-  bandFocused: Band;
+  bandFocused: Band | null;
   scrubPosition: number;
   userMessage: string | null;
 }
@@ -214,3 +354,62 @@ export interface DisplayState {
   zIndex: number;
   dummyVizData: null; // TODO: Figure out what kind of data this actually is.
 }
+
+export interface EqualizerState {
+  on: boolean;
+  auto: boolean;
+  sliders: Record<Slider, number>;
+}
+
+export interface Window {
+  title: string;
+  size: [number, number];
+  open: boolean;
+  hidden: boolean;
+  shade?: boolean;
+  canResize: boolean;
+  canShade: boolean;
+  canDouble: boolean;
+  generic: boolean;
+  hotkey?: string;
+}
+
+export interface WindowState {
+  focused: string;
+  genWindows: { [name: string]: Window };
+  positions: {}; // TODO: Make this more strict
+}
+
+export type MediaTagRequestStatus =
+  | "INITIALIZED"
+  | "FAILED"
+  | "COMPLETE"
+  | "NOT_REQUESTED";
+
+export interface PlaylistTrack {
+  artist: string;
+  title: string;
+  url: string;
+  defaultName: string;
+  albumArtUrl: string | null;
+  selected: boolean;
+  mediaTagsRequestStatus: MediaTagRequestStatus;
+}
+
+export interface PlaylistState {
+  trackOrder: number[];
+  tracks: { [id: number]: PlaylistTrack };
+  lastSelectedIndex: number | null;
+  currentTrack: number | null;
+}
+
+export interface AppState {
+  userInput: UserInputState;
+  windows: WindowState;
+  display: DisplayState;
+  settings: SettingsState;
+  equalizer: EqualizerState;
+  playlist: PlaylistState;
+  media: MediaState;
+  network: NetworkState;
+}