diff --git a/src/actions/editor.ts b/src/actions/editor.ts
new file mode 100644
index 00000000..829382af
--- /dev/null
+++ b/src/actions/editor.ts
@@ -0,0 +1,23 @@
+import { Ace } from 'ace-builds';
+import { Action } from 'redux';
+
+export enum EditorActionType {
+    /**
+     * The current (active) editor changed.
+     */
+    Current = 'editor.action.current',
+}
+
+export interface CurrentEditorAction extends Action<EditorActionType.Current> {
+    editSession: Ace.EditSession | undefined;
+}
+
+/**
+ * Sets the current (active) edit session.
+ * @param editSession The new edit session.
+ */
+export function setEditSession(
+    editSession: Ace.EditSession | undefined,
+): CurrentEditorAction {
+    return { type: EditorActionType.Current, editSession };
+}
diff --git a/src/actions/hub.ts b/src/actions/hub.ts
new file mode 100644
index 00000000..d3b765d2
--- /dev/null
+++ b/src/actions/hub.ts
@@ -0,0 +1,79 @@
+import { Action } from 'redux';
+import { ThunkAction } from 'redux-thunk';
+import { getChecksum } from '../epics/hub';
+import { write } from './ble';
+
+export type HubThunkAction = ThunkAction<Promise<void>, {}, {}, HubRuntimeStatusAction>;
+
+export enum HubRuntimeStatusType {
+    Disconnected = 'disconnected',
+    Idle = 'idle',
+    Loading = 'loading',
+    Loaded = 'loaded',
+    Running = 'running',
+    Error = 'error',
+}
+
+export enum HubActionType {
+    /**
+     * MicroPython runtime status changed.
+     */
+    RuntimeStatus = 'hub.runtime.status',
+    /**
+     * The hub has sent a checksum.
+     */
+    Checksum = 'hub.runtime.checksum',
+}
+
+export interface HubRuntimeStatusAction extends Action<HubActionType.RuntimeStatus> {
+    readonly newStatus: HubRuntimeStatusType;
+}
+
+export interface HubChecksumAction extends Action<HubActionType.Checksum> {
+    readonly checksum: number;
+}
+
+export function updateStatus(newStatus: HubRuntimeStatusType): HubRuntimeStatusAction {
+    return {
+        type: HubActionType.RuntimeStatus,
+        newStatus,
+    };
+}
+
+export function checksum(checksum: number): HubChecksumAction {
+    return {
+        type: HubActionType.Checksum,
+        checksum,
+    };
+}
+
+const downloadChunkSize = 100;
+
+export function downloadAndRun(data: ArrayBuffer): HubThunkAction {
+    return async function (dispatch): Promise<void> {
+        // let everyone know the runtime is busy loading the program
+        dispatch(updateStatus(HubRuntimeStatusType.Loading));
+
+        // TODO: might need to flush checksum queue here
+
+        // first send payload size as big-endian 32-bit integer
+        const sizeBuf = new Uint8Array(4);
+        const sizeView = new DataView(sizeBuf.buffer);
+        sizeView.setUint32(0, data.byteLength);
+        await dispatch(write(sizeBuf));
+
+        // Then send payload in 100 byte chunks waiting for checksum after
+        // each chunk
+        for (let i = 0; i < data.byteLength; i += downloadChunkSize) {
+            // need to subscribe to checksum before writing to prevent race condition
+            const checksum = getChecksum();
+            await dispatch(write(data.slice(i, i + downloadChunkSize)));
+            // TODO: verify checksum
+            console.log(await checksum);
+            // TODO: dispatch progress
+        }
+
+        // let everyone know the runtime is done loading the program
+        dispatch(updateStatus(HubRuntimeStatusType.Loaded));
+    };
+}
diff --git a/src/actions/mpy.ts b/src/actions/mpy.ts
new file mode 100644
index 00000000..8a21aa71
--- /dev/null
+++ b/src/actions/mpy.ts
@@ -0,0 +1,22 @@
+import MpyCross from '@pybricks/mpy-cross';
+import { Action } from 'redux';
+
+// this starts the mpy-cross wasm runtime and leaves it running in the background
+const mpy = MpyCross({ arguments: ['-mno-unicode'] });
+
+enum MpyActionType {
+    Compiled = 'mpy.action.compile',
+}
+
+interface MpyCompiledAction extends Action<MpyActionType.Compiled> {
+    /**
+     * The compiled .mpy data.
+     */
+    data: Uint8Array;
+}
+
+export function compile(script: string): MpyCompiledAction {
+    // TODO: figure out how to capture stderr and emit error action on failure
+    const data = mpy.compile(script);
+    return { type: MpyActionType.Compiled, data };
+}
diff --git a/src/components/ActionButton.tsx b/src/components/ActionButton.tsx
index b3c60ece..381660fb 100644
--- a/src/components/ActionButton.tsx
+++ b/src/components/ActionButton.tsx
@@ -4,7 +4,7 @@ import Tooltip from 'react-bootstrap/Tooltip';
 import Image from 'react-bootstrap/Image';
 import React from 'react';
 
-interface ActionButtonProps {
+export interface ActionButtonProps<T = undefined> {
     /** A unique id for each instance. */
     readonly id: string;
     /** Tooltip text that appears when hovering over the button. */
@@ -14,12 +14,12 @@ interface ActionButtonProps {
     /** When true or undefined, the button is enabled. */
     readonly enabled?: boolean;
     /** Optional action hint passed to the onAction() callback. */
-    readonly action?: string;
+    readonly context?: T;
     /** Callback that is called when the button is activated (clicked). */
-    readonly onAction: (action?: string) => void;
+    readonly onAction: (context?: T) => void;
 }
 
-class ActionButton extends React.Component<ActionButtonProps> {
+class ActionButton<T = undefined> extends React.Component<ActionButtonProps<T>> {
     render(): JSX.Element {
         return (
             <OverlayTrigger
@@ -32,7 +32,7 @@ class ActionButton extends React.Component<ActionButtonProps> {
             >
                 <Button
                     variant="primary"
-                    onClick={(): void => this.props.onAction(this.props.action)}
+                    onClick={(): void => this.props.onAction(this.props.context)}
                     disabled={this.props.enabled === false}
                     style={
                         this.props.enabled === false
diff --git a/src/components/BluetoothButton.tsx b/src/components/BluetoothButton.tsx
index 81f1cb2e..63f8a7da 100644
--- a/src/components/BluetoothButton.tsx
+++ b/src/components/BluetoothButton.tsx
@@ -4,42 +4,35 @@ import { AnyAction } from 'redux';
 import { connect as bleConnect, disconnect as bleDisconnect } from '../actions/ble';
 import { RootState } from '../reducers';
 import { BLEConnectionState } from '../reducers/ble';
-import ActionButton from './ActionButton';
+import ActionButton, { ActionButtonProps } from './ActionButton';
 
 type Dispatch = ThunkDispatch<{}, {}, AnyAction>;
 
-interface StateProps {
-    readonly tooltip: string;
-    readonly icon: string;
-    readonly action?: string;
-    readonly enabled?: boolean;
-}
-
-interface DispatchProps {
-    readonly onAction: (action?: string) => void;
-}
+type ButtonProps = ActionButtonProps<string>;
+type StateProps = Pick<ButtonProps, 'tooltip' | 'icon' | 'context' | 'enabled'>;
+type DispatchProps = Pick<ButtonProps, 'onAction'>;
 
 const mapStateToProps = (state: RootState): StateProps => {
     if (state.ble.connection === BLEConnectionState.Disconnected) {
         return {
             tooltip: 'Connect using Bluetooth',
             icon: 'btdisconnected.svg',
-            action: 'connect',
+            context: 'connect',
             enabled: true,
         };
     } else {
         return {
             tooltip: 'Disconnect Bluetooth',
             icon: 'btconnected.svg',
-            action: 'disconnect',
+            context: 'disconnect',
             enabled: state.ble.connection === BLEConnectionState.Connected,
         };
     }
 };
 
 const mapDispatchToProps = (dispatch: Dispatch): DispatchProps => ({
-    onAction: (a): void => {
-        if (a === 'connect') {
+    onAction: (c): void => {
+        if (c === 'connect') {
             dispatch(bleConnect());
         } else {
             dispatch(bleDisconnect());
diff --git a/src/components/Editor.tsx b/src/components/Editor.tsx
index 4a970131..5a5a5cc2 100644
--- a/src/components/Editor.tsx
+++ b/src/components/Editor.tsx
@@ -1,7 +1,9 @@
-import React from 'react';
+import React, { ReactElement } from 'react';
+import { ReactReduxContext } from 'react-redux';
 import AceEditor from 'react-ace';
 import Row from 'react-bootstrap/Row';
 import Col from 'react-bootstrap/Col';
+import { setEditSession } from '../actions/editor';
 
 import 'ace-builds/src-noconflict/mode-python';
 import 'ace-builds/src-noconflict/theme-xcode';
@@ -13,21 +15,32 @@ class Editor extends React.Component {
         return (
             <Row className="px-2 py-4 bg-primary">
                 <Col>
-                    <AceEditor
-                        mode="python"
-                        theme="xcode"
-                        fontSize="16pt"
-                        width="100"
-                        focus={true}
-                        placeholder="Write your program here..."
-                        defaultValue={localStorage.getItem('program') || undefined}
-                        editorProps={{ $blockScrolling: true }}
-                        setOptions={{
-                            enableBasicAutocompletion: true,
-                            enableLiveAutocompletion: true,
-                        }}
-                        onChange={(v): void => localStorage.setItem('program', v)}
-                    />
+                    <ReactReduxContext.Consumer>
+                        {({ store }): ReactElement => (
+                            <AceEditor
+                                mode="python"
+                                theme="xcode"
+                                fontSize="16pt"
+                                width="100"
+                                focus={true}
+                                placeholder="Write your program here..."
+                                defaultValue={
+                                    localStorage.getItem('program') || undefined
+                                }
+                                editorProps={{ $blockScrolling: true }}
+                                setOptions={{
+                                    enableBasicAutocompletion: true,
+                                    enableLiveAutocompletion: true,
+                                }}
+                                onFocus={(_, e): void => {
+                                    store.dispatch(setEditSession(e?.session));
+                                }}
+                                onChange={(v): void =>
+                                    localStorage.setItem('program', v)
+                                }
+                            />
+                        )}
+                    </ReactReduxContext.Consumer>
                 </Col>
             </Row>
         );
diff --git a/src/components/RunButton.tsx b/src/components/RunButton.tsx
new file mode 100644
index 00000000..83815beb
--- /dev/null
+++ b/src/components/RunButton.tsx
@@ -0,0 +1,50 @@
+import { connect, batch } from 'react-redux';
+import { ThunkDispatch } from 'redux-thunk';
+import { AnyAction } from 'redux';
+import { Ace } from 'ace-builds';
+import { RootState } from '../reducers';
+import { HubRuntimeState } from '../reducers/hub';
+import { compile } from '../actions/mpy';
+import { downloadAndRun } from '../actions/hub';
+import ActionButton, { ActionButtonProps } from './ActionButton';
+
+type Dispatch = ThunkDispatch<{}, {}, AnyAction>;
+
+type ButtonProps = ActionButtonProps<Ace.EditSession>;
+type StateProps = Pick<ButtonProps, 'enabled' | 'context'>;
+type DispatchProps = Pick<ButtonProps, 'onAction'>;
+type OwnProps = Pick<ButtonProps, 'id'>;
+
+const mapStateToProps = (state: RootState): StateProps => ({
+    enabled:
+        state.editor.current !== null && state.hub.runtime === HubRuntimeState.Idle,
+    context: state.editor.current || undefined,
+});
+
+const mapDispatchToProps = (dispatch: Dispatch): DispatchProps => ({
+    onAction: (c): void => {
+        if (!c) {
+            console.error('No current editor');
+            return;
+        }
+        batch(() => {
+            const script = c.getValue();
+            const mpy = dispatch(compile(script));
+            dispatch(downloadAndRun(mpy.data));
+        });
+    },
+});
+
+const mergeProps = (
+    stateProps: StateProps,
+    dispatchProps: DispatchProps,
+    ownProps: OwnProps,
+): ButtonProps => ({
+    tooltip: 'Download and run this program',
+    icon: 'run.svg',
+    ...ownProps,
+    ...stateProps,
+    ...dispatchProps,
+});
+
+export default connect(mapStateToProps, mapDispatchToProps, mergeProps)(ActionButton);
diff --git a/src/components/Toolbar.tsx b/src/components/Toolbar.tsx
index 1aeac128..70d50fef 100644
--- a/src/components/Toolbar.tsx
+++ b/src/components/Toolbar.tsx
@@ -6,6 +6,7 @@ import React from 'react';
 import { BLEConnectionState } from '../reducers/ble';
 import ActionButton from './ActionButton';
 import BluetoothButton from './BluetoothButton';
+import RunButton from './RunButton';
 
 interface ToolbarState {
     bleState: BLEConnectionState;
@@ -33,12 +34,7 @@ class Toolbar extends React.Component<{}, ToolbarState> {
                     <ButtonToolbar className="m-2">
                         <ButtonGroup className="mr-2" size="lg">
                             <BluetoothButton id="bluetooth" />
-                            <ActionButton
-                                id="run"
-                                tooltip="Download and run this program"
-                                icon="run.svg"
-                                onAction={this.onAction}
-                            />
+                            <RunButton id="run" />
                             <ActionButton
                                 id="stop"
                                 tooltip="Stop everything"
diff --git a/src/epics/ble.ts b/src/epics/ble.ts
index 322c79e2..57eaf294 100644
--- a/src/epics/ble.ts
+++ b/src/epics/ble.ts
@@ -1,17 +1,52 @@
 import { combineEpics, Epic, ofType } from 'redux-observable';
 import { map } from 'rxjs/operators';
-import { BLEDataActionType, BLEDataAction } from '../actions/ble';
-import { TerminalDataAction, sendData } from '../actions/terminal';
+import { AnyAction } from 'redux';
+import { BLEDataActionType, BLEDataAction, BLEConnectActionType } from '../actions/ble';
+import { sendData } from '../actions/terminal';
+import { updateStatus, HubRuntimeStatusType, checksum } from '../actions/hub';
+import { RootState } from '../reducers';
+import { HubRuntimeState } from '../reducers/hub';
 
 const decoder = new TextDecoder();
 
-const rxUartData: Epic = (action$) =>
+const connect: Epic = (action$) =>
     action$.pipe(
-        ofType(BLEDataActionType.ReceivedData),
-        map(
-            (a: BLEDataAction): TerminalDataAction =>
-                sendData(decoder.decode(a.value.buffer)),
-        ),
+        ofType(BLEConnectActionType.EndConnect),
+        map(() => updateStatus(HubRuntimeStatusType.Idle)),
     );
 
-export default combineEpics(rxUartData);
+const disconnect: Epic = (action$) =>
+    action$.pipe(
+        ofType(BLEConnectActionType.EndDisconnect),
+        map(() => updateStatus(HubRuntimeStatusType.Disconnected)),
+    );
+
+const rxUartData: Epic<AnyAction, AnyAction, RootState> = (action$, state$) =>
+    action$.pipe(
+        ofType<AnyAction, BLEDataAction>(BLEDataActionType.ReceivedData),
+        map((a) => {
+            if (
+                state$.value.hub.runtime === HubRuntimeState.Loading &&
+                a.value.buffer.byteLength === 1
+            ) {
+                const view = new DataView(a.value.buffer);
+                return checksum(view.getUint8(0));
+            } else {
+                const value = decoder.decode(a.value.buffer);
+                // FIXME: sometimes we get ERROR and IDLE in same message except
+                // last E is cut off
+                if (value.match(/>>>> IDLE/)) {
+                    return updateStatus(HubRuntimeStatusType.Idle);
+                }
+                if (value.match(/>>>> ERROR/)) {
+                    return updateStatus(HubRuntimeStatusType.Error);
+                }
+                if (value.match(/>>>> RUNNING/)) {
+                    return updateStatus(HubRuntimeStatusType.Running);
+                }
+                return sendData(value);
+            }
+        }),
+    );
+
+export default combineEpics(connect, disconnect, rxUartData);
diff --git a/src/epics/hub.ts b/src/epics/hub.ts
new file mode 100644
index 00000000..3e55c870
--- /dev/null
+++ b/src/epics/hub.ts
@@ -0,0 +1,20 @@
+import { combineEpics, Epic, ofType } from 'redux-observable';
+import { AnyAction } from 'redux';
+import { take, tap, ignoreElements } from 'rxjs/operators';
+import { Subject } from 'rxjs';
+import { HubActionType, HubChecksumAction } from '../actions/hub';
+
+const checksumSubject = new Subject<number>();
+
+const checksum: Epic = (action$) =>
+    action$.pipe(
+        ofType<AnyAction, HubChecksumAction>(HubActionType.Checksum),
+        tap((a) => checksumSubject.next(a.checksum)),
+        ignoreElements(),
+    );
+
+export function getChecksum(): Promise<number> {
+    return checksumSubject.pipe(take(1)).toPromise();
+}
+
+export default combineEpics(checksum);
diff --git a/src/epics/index.ts b/src/epics/index.ts
index 44dbc556..71215751 100644
--- a/src/epics/index.ts
+++ b/src/epics/index.ts
@@ -1,10 +1,11 @@
 import { combineEpics, Epic } from 'redux-observable';
 import { catchError } from 'rxjs/operators';
 import ble from './ble';
+import hub from './hub';
 import terminal from './terminal';
 
 const rootEpic: Epic = (action$, store$, dependencies) =>
-    combineEpics(ble, terminal)(action$, store$, dependencies).pipe(
+    combineEpics(ble, hub, terminal)(action$, store$, dependencies).pipe(
         catchError((error, source) => {
             console.error(error);
             return source;
diff --git a/src/epics/terminal.ts b/src/epics/terminal.ts
index 88af9926..abc8770d 100644
--- a/src/epics/terminal.ts
+++ b/src/epics/terminal.ts
@@ -1,7 +1,8 @@
 import { combineEpics, Epic, ofType } from 'redux-observable';
 import { Subject } from 'rxjs';
 import { map, tap, ignoreElements } from 'rxjs/operators';
-import { write, BLEThunkAction } from '../actions/ble';
+import { AnyAction } from 'redux';
+import { write } from '../actions/ble';
 import { TerminalDataAction, TerminalDataActionType } from '../actions/terminal';
 
 const encoder = new TextEncoder();
@@ -12,16 +13,16 @@ export const terminalOutput = terminalOutputSubject.asObservable();
 // When terminal has focus this receives the input. All input is sent over BLE connection.
 const receiveTerminalData: Epic = (action$) =>
     action$.pipe(
-        ofType(TerminalDataActionType.ReceivedData),
-        map((a: TerminalDataAction): BLEThunkAction => write(encoder.encode(a.value))),
+        ofType<AnyAction, TerminalDataAction>(TerminalDataActionType.ReceivedData),
+        map((a) => write(encoder.encode(a.value))),
     );
 
 // Request to write to the terminal are handled by an observable rather than
 // using redux state.
 const sendTerminalData: Epic = (action$) =>
     action$.pipe(
-        ofType(TerminalDataActionType.SendData),
-        tap((a: TerminalDataAction): void => terminalOutputSubject.next(a.value)),
+        ofType<AnyAction, TerminalDataAction>(TerminalDataActionType.SendData),
+        tap((a) => terminalOutputSubject.next(a.value)),
         ignoreElements(),
     );
 
diff --git a/src/reducers/ble.ts b/src/reducers/ble.ts
index ed0c8f96..4a1663cc 100644
--- a/src/reducers/ble.ts
+++ b/src/reducers/ble.ts
@@ -23,13 +23,6 @@ export enum BLEConnectionState {
     Disconnecting = 'ble.connection.disconnecting',
 }
 
-/**
- * BLE state for redux store.
- */
-export interface BLEState {
-    readonly connection: BLEConnectionState;
-}
-
 const connection: Reducer<BLEConnectionState> = (
     state = BLEConnectionState.Disconnected,
     action,
@@ -48,4 +41,8 @@ const connection: Reducer<BLEConnectionState> = (
     }
 };
 
+export interface BLEState {
+    readonly connection: BLEConnectionState;
+}
+
 export default combineReducers({ connection });
diff --git a/src/reducers/editor.ts b/src/reducers/editor.ts
new file mode 100644
index 00000000..0544e16c
--- /dev/null
+++ b/src/reducers/editor.ts
@@ -0,0 +1,22 @@
+import { combineReducers, Reducer } from 'redux';
+import { Ace } from 'ace-builds';
+import { CurrentEditorAction, EditorActionType } from '../actions/editor';
+
+type CurrentEditSession = Ace.EditSession | null;
+
+const current: Reducer<CurrentEditSession, CurrentEditorAction> = (
+    state = null,
+    action,
+) => {
+    switch (action.type) {
+        case EditorActionType.Current:
+            return action.editSession || null;
+        default:
+            return state;
+    }
+};
+
+export interface EditorState {
+    current: CurrentEditSession;
+}
+export default combineReducers({ current });
diff --git a/src/reducers/hub.ts b/src/reducers/hub.ts
new file mode 100644
index 00000000..8a0327c1
--- /dev/null
+++ b/src/reducers/hub.ts
@@ -0,0 +1,70 @@
+import { Reducer, combineReducers } from 'redux';
+import {
+    HubRuntimeStatusAction,
+    HubActionType,
+    HubRuntimeStatusType,
+} from '../actions/hub';
+
+/**
+ * Describes the state of the MicroPython runtime on the hub.
+ */
+export enum HubRuntimeState {
+    /**
+     * The hub is not connected.
+     */
+    Disconnected = 'hub.runtime.disconnected',
+    /**
+     * The runtime is idle waiting for command after soft reboot.
+     */
+    Idle = 'hub.runtime.idle',
+    /**
+     * A user program is being copied to the hub.
+     */
+    Loading = 'hub.runtime.loading',
+    /**
+     * A user program has been copied to the hub.
+     */
+    Loaded = 'hub.runtime.loaded',
+    /**
+     * A user program is running.
+     */
+    Running = 'hub.runtime.running',
+    /**
+     * The runtime encountered an error.
+     */
+    Error = 'hub.runtime.error',
+}
+
+const runtime: Reducer<HubRuntimeState, HubRuntimeStatusAction> = (
+    state = HubRuntimeState.Disconnected,
+    action,
+) => {
+    switch (action.type) {
+        case HubActionType.RuntimeStatus:
+            switch (action.newStatus) {
+                case HubRuntimeStatusType.Disconnected:
+                    return HubRuntimeState.Disconnected;
+                case HubRuntimeStatusType.Idle:
+                    return HubRuntimeState.Idle;
+                case HubRuntimeStatusType.Loading:
+                    return HubRuntimeState.Loading;
+                case HubRuntimeStatusType.Loaded:
+                    return HubRuntimeState.Loaded;
+                case HubRuntimeStatusType.Running:
+                    return HubRuntimeState.Running;
+                case HubRuntimeStatusType.Error:
+                    return HubRuntimeState.Error;
+                default:
+                    console.error(`bad action/state: ${action.newStatus}`);
+                    return state;
+            }
+        default:
+            return state;
+    }
+};
+
+export interface HubState {
+    readonly runtime: HubRuntimeState;
+}
+
+export default combineReducers({ runtime });
diff --git a/src/reducers/index.ts b/src/reducers/index.ts
index 3c00b56d..6d0ed698 100644
--- a/src/reducers/index.ts
+++ b/src/reducers/index.ts
@@ -1,11 +1,15 @@
 import { combineReducers } from 'redux';
 import ble, { BLEState } from './ble';
+import editor, { EditorState } from './editor';
+import hub, { HubState } from './hub';
 
 /**
  * Root state for redux store.
  */
 export interface RootState {
     readonly ble: BLEState;
+    readonly editor: EditorState;
+    readonly hub: HubState;
 }
 
-export default combineReducers({ ble });
+export default combineReducers({ ble, editor, hub });