diff --git a/commands/document.test.ts b/commands/document.test.ts
index 1ba4574..1dc7c11 100644
--- a/commands/document.test.ts
+++ b/commands/document.test.ts
@@ -18,6 +18,7 @@ import {
   documentSetLayerClippingMaskCommand,
   documentSetLayerLockedCommand,
   documentSetLayerVisibleCommand,
+  documentUpdateAssetSourceCommand,
   documentUngroupLayerCommand,
 } from "./document";
 
@@ -52,6 +53,17 @@ describe("document commands", () => {
     ]);
   });
 
+  test("updates asset sources", () => {
+    const state = documentAddAssetCommand.execute(
+      { state: createInitialAppState("Test") },
+      { asset: { id: "asset-1", name: "Image", mimeType: "image/png", source: "old", intrinsicSize: { w: 100, h: 50 } } },
+    );
+
+    const next = documentUpdateAssetSourceCommand.execute({ state }, { assetId: "asset-1", source: "new" });
+
+    expect(next.document.assets[0]?.source).toBe("new");
+  });
+
   test("adds image layers to artboards", () => {
     const state = documentAddArtboardCommand.execute(
       { state: createInitialAppState("Test") },
diff --git a/commands/document.ts b/commands/document.ts
index a735f68..3e181a9 100644
--- a/commands/document.ts
+++ b/commands/document.ts
@@ -1,7 +1,7 @@
 import type { Asset } from "@core/asset";
 import type { ImageDocument } from "@core/document";
 import type { Rect } from "@core/geometry";
-import type { ArtboardId, LayerId } from "@core/id";
+import type { ArtboardId, AssetId, LayerId } from "@core/id";
 import type { ImageLayer } from "@core/image-layer";
 import type { Layer } from "@core/layer";
 import type { RasterLayer } from "@core/raster-layer";
@@ -43,6 +43,11 @@ export type DocumentAddAssetPayload = {
   asset: Asset;
 };
 
+export type DocumentUpdateAssetSourcePayload = {
+  assetId: AssetId;
+  source: string;
+};
+
 export type DocumentAddImageLayerPayload = {
   artboardId: ArtboardId;
   parentGroupId?: LayerId;
@@ -219,6 +224,20 @@ export const documentAddAssetCommand: Command<DocumentAddAssetPayload> = {
   },
 };
 
+export const documentUpdateAssetSourceCommand: Command<DocumentUpdateAssetSourcePayload> = {
+  id: commandIds.documentUpdateAssetSource,
+  name: "Update asset source",
+  execute({ state }, payload) {
+    return {
+      ...state,
+      document: {
+        ...state.document,
+        assets: state.document.assets.map((asset) => (asset.id === payload.assetId ? { ...asset, source: payload.source } : asset)),
+      },
+    };
+  },
+};
+
 export const documentAddImageLayerCommand: Command<DocumentAddImageLayerPayload> = {
   id: commandIds.documentAddImageLayer,
   name: "Add image layer",
@@ -406,6 +425,7 @@ export const documentCommands = [
   documentSetArtboardLockedCommand,
   documentRenameArtboardCommand,
   documentAddAssetCommand,
+  documentUpdateAssetSourceCommand,
   documentAddImageLayerCommand,
   documentAddRasterLayerCommand,
   documentAddGroupLayerCommand,
diff --git a/commands/ids.ts b/commands/ids.ts
index 1777619..0fe5bf5 100644
--- a/commands/ids.ts
+++ b/commands/ids.ts
@@ -6,6 +6,7 @@ export const commandIds = {
   documentSetArtboardLocked: "document.setArtboardLocked",
   documentRenameArtboard: "document.renameArtboard",
   documentAddAsset: "document.addAsset",
+  documentUpdateAssetSource: "document.updateAssetSource",
   documentAddImageLayer: "document.addImageLayer",
   documentAddRasterLayer: "document.addRasterLayer",
   documentAddGroupLayer: "document.addGroupLayer",
diff --git a/commands/index.ts b/commands/index.ts
index 39a02e2..5ad9c7c 100644
--- a/commands/index.ts
+++ b/commands/index.ts
@@ -18,6 +18,7 @@ export {
   documentSetLayerClippingMaskCommand,
   documentSetLayerLockedCommand,
   documentSetLayerVisibleCommand,
+  documentUpdateAssetSourceCommand,
   documentUngroupLayerCommand,
 } from "./document";
 export type {
@@ -38,6 +39,7 @@ export type {
   DocumentSetLayerClippingMaskPayload,
   DocumentSetLayerLockedPayload,
   DocumentSetLayerVisiblePayload,
+  DocumentUpdateAssetSourcePayload,
   DocumentUngroupLayerPayload,
 } from "./document";
 export { historyCommands, historyRedoCommand, historyUndoCommand } from "./history";
diff --git a/commands/payloads.ts b/commands/payloads.ts
index 4f88253..92f71b0 100644
--- a/commands/payloads.ts
+++ b/commands/payloads.ts
@@ -17,6 +17,7 @@ import type {
   DocumentSetLayerClippingMaskPayload,
   DocumentSetLayerLockedPayload,
   DocumentSetLayerVisiblePayload,
+  DocumentUpdateAssetSourcePayload,
   DocumentUngroupLayerPayload,
 } from "./document";
 import type { SelectionAddLayerPayload, SelectionSetPayload } from "./selection";
@@ -38,6 +39,7 @@ export type CommandPayloads = {
   [commandIds.documentSetArtboardLocked]: DocumentSetArtboardLockedPayload;
   [commandIds.documentRenameArtboard]: DocumentRenameArtboardPayload;
   [commandIds.documentAddAsset]: DocumentAddAssetPayload;
+  [commandIds.documentUpdateAssetSource]: DocumentUpdateAssetSourcePayload;
   [commandIds.documentAddImageLayer]: DocumentAddImageLayerPayload;
   [commandIds.documentAddRasterLayer]: DocumentAddRasterLayerPayload;
   [commandIds.documentAddGroupLayer]: DocumentAddGroupLayerPayload;
diff --git a/editor/tools.ts b/editor/tools.ts
index a8407ec..f0c2416 100644
--- a/editor/tools.ts
+++ b/editor/tools.ts
@@ -1,4 +1,4 @@
-export const availableToolIds = ["select", "crop", "pan"] as const;
+export const availableToolIds = ["select", "crop", "brush", "pan"] as const;
 
 export type ToolId = (typeof availableToolIds)[number];
 
diff --git a/view/ToolOverlay.tsx b/view/ToolOverlay.tsx
index 9caeb76..d694165 100644
--- a/view/ToolOverlay.tsx
+++ b/view/ToolOverlay.tsx
@@ -1,4 +1,4 @@
-import { Crop, Cursor, Hand } from "@phosphor-icons/react";
+import { Crop, Cursor, Hand, PaintBrush } from "@phosphor-icons/react";
 import { commandIds } from "@commands/ids";
 import type { AppStore } from "@editor/store";
 import type { InteractionMode, ToolId } from "@editor/tools";
@@ -43,6 +43,8 @@ function iconForTool(tool: ToolId) {
   switch (tool) {
     case "crop":
       return Crop;
+    case "brush":
+      return PaintBrush;
     case "pan":
       return Hand;
     case "select":
diff --git a/view/canvas/brush.ts b/view/canvas/brush.ts
new file mode 100644
index 0000000..dba8b27
--- /dev/null
+++ b/view/canvas/brush.ts
@@ -0,0 +1,105 @@
+import { commandIds } from "@commands/ids";
+import type { ImageDocument } from "@core/document";
+import type { Vec2D } from "@core/geometry";
+import type { Layer } from "@core/layer";
+import type { RasterLayer } from "@core/raster-layer";
+import type { EditorState } from "@editor/state";
+import type { AppStore } from "@editor/store";
+
+export type BrushSession = {
+  layerId: string;
+  previousPoint: Vec2D;
+};
+
+export function beginBrushSession(document: ImageDocument, editor: EditorState, point: Vec2D): BrushSession | undefined {
+  if (editor.tools.activeTool !== "brush") return undefined;
+  const layerId = editor.selection.layerIds[0];
+  if (!layerId) return undefined;
+  const layer = findRasterLayer(document.artboards.flatMap((artboard) => artboard.layers), layerId);
+  if (!layer || layer.locked || !layer.visible) return undefined;
+  return { layerId, previousPoint: point };
+}
+
+export async function updateBrushSession(options: {
+  store: AppStore;
+  session: BrushSession;
+  point: Vec2D;
+  color?: string;
+  size?: number;
+}): Promise<BrushSession> {
+  const state = options.store.getState();
+  const layer = findRasterLayer(state.document.artboards.flatMap((artboard) => artboard.layers), options.session.layerId);
+  if (!layer) return { ...options.session, previousPoint: options.point };
+
+  const asset = state.document.assets.find((candidate) => candidate.id === layer.assetId);
+  if (!asset) return { ...options.session, previousPoint: options.point };
+
+  const source = await drawStroke({
+    source: asset.source,
+    width: asset.intrinsicSize.w,
+    height: asset.intrinsicSize.h,
+    from: documentPointToAssetPoint(options.session.previousPoint, layer, asset.intrinsicSize.w, asset.intrinsicSize.h),
+    to: documentPointToAssetPoint(options.point, layer, asset.intrinsicSize.w, asset.intrinsicSize.h),
+    color: options.color ?? "#111827",
+    size: options.size ?? 8,
+  });
+
+  options.store.dispatch(commandIds.documentUpdateAssetSource, { assetId: asset.id, source });
+  return { ...options.session, previousPoint: options.point };
+}
+
+function documentPointToAssetPoint(point: Vec2D, layer: RasterLayer, width: number, height: number): Vec2D {
+  return {
+    x: ((point.x - layer.transform.position.x) / Math.max(0.0001, layer.transform.scale.x) / width) * width,
+    y: ((point.y - layer.transform.position.y) / Math.max(0.0001, layer.transform.scale.y) / height) * height,
+  };
+}
+
+async function drawStroke(options: {
+  source: string;
+  width: number;
+  height: number;
+  from: Vec2D;
+  to: Vec2D;
+  color: string;
+  size: number;
+}) {
+  const canvas = document.createElement("canvas");
+  canvas.width = Math.max(1, Math.round(options.width));
+  canvas.height = Math.max(1, Math.round(options.height));
+  const context = canvas.getContext("2d");
+  if (!context) return options.source;
+
+  const image = await loadImage(options.source);
+  context.drawImage(image, 0, 0, canvas.width, canvas.height);
+  context.strokeStyle = options.color;
+  context.lineWidth = options.size;
+  context.lineCap = "round";
+  context.lineJoin = "round";
+  context.beginPath();
+  context.moveTo(options.from.x, options.from.y);
+  context.lineTo(options.to.x, options.to.y);
+  context.stroke();
+
+  return canvas.toDataURL("image/png");
+}
+
+function loadImage(source: string) {
+  return new Promise<HTMLImageElement>((resolve, reject) => {
+    const image = new Image();
+    image.onload = () => resolve(image);
+    image.onerror = () => reject(new Error("Failed to load raster layer"));
+    image.src = source;
+  });
+}
+
+function findRasterLayer(layers: Layer[], layerId: string): RasterLayer | undefined {
+  for (const layer of layers) {
+    if (layer.id === layerId && layer.type === "raster") return layer;
+    if (layer.type === "group") {
+      const child = findRasterLayer(layer.children, layerId);
+      if (child) return child;
+    }
+  }
+  return undefined;
+}
diff --git a/view/canvas/cursor.ts b/view/canvas/cursor.ts
index 82c7b4e..abf9a42 100644
--- a/view/canvas/cursor.ts
+++ b/view/canvas/cursor.ts
@@ -5,6 +5,6 @@ import type { CanvasInputState } from "./useCanvasInput";
 export function canvasCursorClass(interactionMode: InteractionMode, input: CanvasInputState) {
   if (input.isPanning) return "cursor-grabbing";
   if (isPanInteractionMode(interactionMode)) return "cursor-grab";
-  if (interactionMode.type === "tool" && interactionMode.tool === "crop") return "cursor-crosshair";
+  if (interactionMode.type === "tool" && (interactionMode.tool === "crop" || interactionMode.tool === "brush")) return "cursor-crosshair";
   return "cursor-default";
 }
diff --git a/view/canvas/useCanvasInput.ts b/view/canvas/useCanvasInput.ts
index f3ab340..7f16b5b 100644
--- a/view/canvas/useCanvasInput.ts
+++ b/view/canvas/useCanvasInput.ts
@@ -1,4 +1,4 @@
-import { useEffect, useState, type RefObject } from "react";
+import { useEffect, useRef, useState, type RefObject } from "react";
 import type { AppStore } from "@editor/store";
 import { isPanInteractionMode } from "@editor/tools";
 import type { GlobalKeybindConsumer, GlobalPointerConsumer, GlobalWheelConsumer } from "@input/index";
@@ -11,6 +11,7 @@ import {
   pointerInputEventFromPointerEvent,
   wheelInputEventFromWheelEvent,
 } from "@input/index";
+import { beginBrushSession, updateBrushSession, type BrushSession } from "./brush";
 
 export type CanvasInputOptions = {
   globalKeybindConsumer: GlobalKeybindConsumer;
@@ -28,6 +29,7 @@ export function useCanvasInput(
   options: CanvasInputOptions,
 ): CanvasInputState {
   const [isPanning, setIsPanning] = useState(false);
+  const brushSession = useRef<BrushSession>();
 
   useEffect(() => {
     const canvas = canvasRef.current;
@@ -59,6 +61,15 @@ export function useCanvasInput(
 
     const handlePointerDown = (event: PointerEvent) => {
       const inputEvent = pointerInputEventFromPointerEvent(event);
+      const state = store.getState();
+      const brush = beginBrushSession(state.document, state.editor, viewportPointToDocumentPoint(inputEvent.position, state.editor.viewport));
+      if (brush) {
+        brushSession.current = brush;
+        canvas.setPointerCapture(event.pointerId);
+        event.preventDefault();
+        return;
+      }
+
       const transformed = transformHandler.pointerDown(inputEvent);
       if (transformed) {
         canvas.setPointerCapture(event.pointerId);
@@ -74,12 +85,12 @@ export function useCanvasInput(
         return;
       }
 
-      const state = store.getState();
-      const selectionToolActive = state.editor.tools.activeTool === "select" || state.editor.tools.activeTool === "crop";
+      const currentState = store.getState();
+      const selectionToolActive = currentState.editor.tools.activeTool === "select" || currentState.editor.tools.activeTool === "crop";
       const selected = selectionToolActive && handleArtboardSelection({
         event: inputEvent,
-        document: state.document,
-        viewport: state.editor.viewport,
+        document: currentState.document,
+        viewport: currentState.editor.viewport,
         dispatch: store.dispatch,
       });
       if (selected) event.preventDefault();
@@ -87,6 +98,15 @@ export function useCanvasInput(
 
     const handlePointerMove = (event: PointerEvent) => {
       const inputEvent = pointerInputEventFromPointerEvent(event);
+      if (brushSession.current) {
+        const point = viewportPointToDocumentPoint(inputEvent.position, store.getState().editor.viewport);
+        void updateBrushSession({ store, session: brushSession.current, point }).then((nextSession) => {
+          brushSession.current = nextSession;
+        });
+        event.preventDefault();
+        return;
+      }
+
       const transformed = transformHandler.pointerMove(inputEvent);
       if (transformed) {
         event.preventDefault();
@@ -99,6 +119,12 @@ export function useCanvasInput(
 
     const handlePointerUp = (event: PointerEvent) => {
       const inputEvent = pointerInputEventFromPointerEvent(event);
+      if (brushSession.current) {
+        brushSession.current = undefined;
+        event.preventDefault();
+        return;
+      }
+
       const transformed = transformHandler.pointerUp(inputEvent);
       if (transformed) {
         event.preventDefault();
@@ -144,3 +170,10 @@ export function useCanvasInput(
 
   return { isPanning };
 }
+
+function viewportPointToDocumentPoint(point: { x: number; y: number }, viewport: { center: { x: number; y: number }; size: { w: number; h: number }; zoom: number }) {
+  return {
+    x: viewport.center.x + (point.x - viewport.size.w / 2) / viewport.zoom,
+    y: viewport.center.y + (point.y - viewport.size.h / 2) / viewport.zoom,
+  };
+}
diff --git a/view/toolLabels.ts b/view/toolLabels.ts
index fcfaabe..a27872f 100644
--- a/view/toolLabels.ts
+++ b/view/toolLabels.ts
@@ -4,6 +4,8 @@ export function labelForTool(tool: ToolId): string {
   switch (tool) {
     case "crop":
       return "Crop";
+    case "brush":
+      return "Brush";
     case "pan":
       return "Pan";
     case "select":