diff --git a/e2e/save.spec.ts b/e2e/save.spec.ts
new file mode 100644
index 0000000..499b158
--- /dev/null
+++ b/e2e/save.spec.ts
@@ -0,0 +1,195 @@
+import { test, expect, type Page, type Download } from '@playwright/test';
+import * as fs from 'fs';
+
+// ── types mirroring saveGraph.ts ──────────────────────────────────────────────
+interface SavedEdge { id: string; from: string; to: string; }
+interface SavedNode { id: string; label?: string; subgraph?: SavedGraph; }
+interface SavedGraph { id: string; nodes: SavedNode[]; edges: SavedEdge[]; }
+
+// ── helpers ───────────────────────────────────────────────────────────────────
+async function waitForGraph(page: Page) {
+  await page.waitForSelector('svg g.node', { timeout: 15000 });
+}
+
+function nodeByLabel(page: Page, label: string) {
+  return page.locator('g.node').filter({ hasText: label });
+}
+
+async function openSubgraph(page: Page, nodeLabel: string) {
+  await nodeByLabel(page, nodeLabel).click({ button: 'right' });
+  await expect(page.locator('.ant-dropdown:visible')).toBeVisible({ timeout: 3000 });
+  await page.waitForTimeout(300);
+  await page.locator('.ant-dropdown-menu-item').filter({ hasText: 'Subgraph' }).click();
+  await waitForGraph(page);
+}
+
+function saveButton(page: Page) {
+  return page.getByTitle('Save as JSON');
+}
+
+async function triggerSave(page: Page): Promise<Download> {
+  const [download] = await Promise.all([
+    page.waitForEvent('download'),
+    saveButton(page).click(),
+  ]);
+  return download;
+}
+
+async function getSavedJson(page: Page): Promise<SavedGraph> {
+  const download = await triggerSave(page);
+  const filePath = await download.path();
+  expect(filePath).not.toBeNull();
+  return JSON.parse(fs.readFileSync(filePath!, 'utf-8')) as SavedGraph;
+}
+
+// ── tests ─────────────────────────────────────────────────────────────────────
+test.describe('Graph saving', () => {
+  test.beforeEach(async ({ page }) => {
+    await page.goto('/');
+    await waitForGraph(page);
+  });
+
+  // ── button presence ─────────────────────────────────────────────────────────
+  test('save button is visible in the header', async ({ page }) => {
+    await expect(saveButton(page)).toBeVisible();
+  });
+
+  // ── download mechanics ──────────────────────────────────────────────────────
+  test('clicking save downloads a file named concept-sketch.json', async ({ page }) => {
+    const download = await triggerSave(page);
+    expect(download.suggestedFilename()).toBe('concept-sketch.json');
+  });
+
+  test('downloaded file is valid JSON', async ({ page }) => {
+    const download = await triggerSave(page);
+    const filePath = await download.path();
+    expect(filePath).not.toBeNull();
+    expect(() => JSON.parse(fs.readFileSync(filePath!, 'utf-8'))).not.toThrow();
+  });
+
+  // ── root graph structure ────────────────────────────────────────────────────
+  test('saved JSON has id, nodes and edges at the root', async ({ page }) => {
+    const data = await getSavedJson(page);
+    expect(data.id).toBe('main');
+    expect(Array.isArray(data.nodes)).toBe(true);
+    expect(Array.isArray(data.edges)).toBe(true);
+  });
+
+  test('default graph saves Start and End nodes with one connecting edge', async ({ page }) => {
+    const data = await getSavedJson(page);
+    expect(data.nodes).toHaveLength(2);
+    const labels = data.nodes.map(n => n.label);
+    expect(labels).toContain('Start');
+    expect(labels).toContain('End');
+    expect(data.edges).toHaveLength(1);
+  });
+
+  test('edge from/to ids reference nodes that exist in the graph', async ({ page }) => {
+    const data = await getSavedJson(page);
+    const nodeIds = new Set(data.nodes.map(n => n.id));
+    for (const edge of data.edges) {
+      expect(nodeIds.has(edge.from)).toBe(true);
+      expect(nodeIds.has(edge.to)).toBe(true);
+    }
+  });
+
+  // ── edits reflected in the save ─────────────────────────────────────────────
+  test('newly added node appears in the saved JSON', async ({ page }) => {
+    await nodeByLabel(page, 'Start').click();
+    await expect(page.locator('g.node')).toHaveCount(3, { timeout: 5000 });
+
+    const data = await getSavedJson(page);
+    expect(data.nodes).toHaveLength(3);
+    expect(data.edges).toHaveLength(2);
+  });
+
+  test('renamed node label appears correctly in the saved JSON', async ({ page }) => {
+    await nodeByLabel(page, 'Start').click({ button: 'right' });
+    await expect(page.locator('.ant-dropdown:visible')).toBeVisible({ timeout: 3000 });
+    await page.locator('.ant-dropdown-menu-item').filter({ hasText: 'Rename' }).click();
+
+    const modal = page.locator('.ant-modal');
+    await expect(modal).toBeVisible({ timeout: 3000 });
+    await modal.locator('.ant-input').clear();
+    await modal.locator('.ant-input').fill('Concept');
+    await modal.locator('.ant-btn-primary').click();
+    await expect(modal).not.toBeVisible();
+    await expect(nodeByLabel(page, 'Concept')).toBeVisible({ timeout: 5000 });
+
+    const data = await getSavedJson(page);
+    const labels = data.nodes.map(n => n.label);
+    expect(labels).toContain('Concept');
+    expect(labels).not.toContain('Start');
+  });
+
+  test('removed node is absent from the saved JSON', async ({ page }) => {
+    await nodeByLabel(page, 'End').click({ button: 'right' });
+    await expect(page.locator('.ant-dropdown:visible')).toBeVisible({ timeout: 3000 });
+    await page.locator('.ant-dropdown-menu-item').filter({ hasText: 'Remove' }).click();
+    await expect(page.locator('g.node')).toHaveCount(1, { timeout: 5000 });
+
+    const data = await getSavedJson(page);
+    expect(data.nodes).toHaveLength(1);
+    expect(data.nodes.map(n => n.label)).not.toContain('End');
+  });
+
+  // ── subgraph serialisation ──────────────────────────────────────────────────
+  test('entering a subgraph and returning adds it nested inside the parent node', async ({ page }) => {
+    await openSubgraph(page, 'Start');
+    await page.locator('.ant-breadcrumb-link').first().click();
+    await waitForGraph(page);
+
+    const data = await getSavedJson(page);
+    const startNode = data.nodes.find(n => n.label === 'Start')!;
+    expect(startNode.subgraph).toBeDefined();
+    expect(startNode.subgraph!.nodes).toHaveLength(2);
+    expect(startNode.subgraph!.edges).toHaveLength(1);
+  });
+
+  test('subgraph id matches the id of the node that contains it', async ({ page }) => {
+    await openSubgraph(page, 'Start');
+    await page.locator('.ant-breadcrumb-link').first().click();
+    await waitForGraph(page);
+
+    const data = await getSavedJson(page);
+    const startNode = data.nodes.find(n => n.label === 'Start')!;
+    expect(startNode.subgraph!.id).toBe(startNode.id);
+  });
+
+  test('node without a subgraph has no subgraph key in the JSON', async ({ page }) => {
+    await openSubgraph(page, 'Start');
+    await page.locator('.ant-breadcrumb-link').first().click();
+    await waitForGraph(page);
+
+    const data = await getSavedJson(page);
+    const endNode = data.nodes.find(n => n.label === 'End')!;
+    expect(endNode.subgraph).toBeUndefined();
+  });
+
+  test('nodes added inside a subgraph are serialised in the nested subgraph', async ({ page }) => {
+    await openSubgraph(page, 'Start');
+    // Add one extra node inside the subgraph
+    await nodeByLabel(page, 'End').click();
+    await expect(page.locator('g.node')).toHaveCount(3, { timeout: 5000 });
+
+    await page.locator('.ant-breadcrumb-link').first().click();
+    await waitForGraph(page);
+
+    const data = await getSavedJson(page);
+    const startNode = data.nodes.find(n => n.label === 'Start')!;
+    expect(startNode.subgraph!.nodes).toHaveLength(3);
+    expect(startNode.subgraph!.edges).toHaveLength(2);
+  });
+
+  test('saving from inside a subgraph includes the full hierarchy from root', async ({ page }) => {
+    await openSubgraph(page, 'Start');
+    // Still inside the subgraph when saving
+    const data = await getSavedJson(page);
+    // Root must still be present
+    expect(data.id).toBe('main');
+    expect(data.nodes.map(n => n.label)).toContain('Start');
+    // And the subgraph must be nested
+    const startNode = data.nodes.find(n => n.label === 'Start')!;
+    expect(startNode.subgraph).toBeDefined();
+  });
+});
diff --git a/src/App.tsx b/src/App.tsx
index 9ba79f2..5bec202 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -5,8 +5,10 @@ import type { BreadcrumbItemType } from 'antd/es/breadcrumb/Breadcrumb';
 import { useKeysdownStore } from './stores/ArrayStore';
 import {
   MenuFoldOutlined,
-  MenuUnfoldOutlined
+  MenuUnfoldOutlined,
+  SaveOutlined,
 } from '@ant-design/icons';
+import { saveConceptSketch } from './utils/saveGraph';
 import { useGraphLayersTreeStore } from './stores/TreeStore';
 
 const { Header, Content, Sider } = Layout;
@@ -67,6 +69,18 @@ const App: React.FC = () => {
                   color: colorBgContainer,
                 }}
               />
+              <Button
+                type="text"
+                icon={<SaveOutlined />}
+                onClick={saveConceptSketch}
+                title="Save as JSON"
+                style={{
+                  fontSize: '16px',
+                  width: 32,
+                  height: 32,
+                  color: colorBgContainer,
+                }}
+              />
             </div>
             <Breadcrumb items={graphLevel} />
           </Space>
diff --git a/src/utils/saveGraph.ts b/src/utils/saveGraph.ts
new file mode 100644
index 0000000..728b2bc
--- /dev/null
+++ b/src/utils/saveGraph.ts
@@ -0,0 +1,53 @@
+import type { GraphModel } from '../components/Graph';
+import { useGraphsStore } from '../stores/GraphsStore';
+
+interface SerializedEdge {
+  id: string;
+  from: string;
+  to: string;
+}
+
+interface SerializedNode {
+  id: string;
+  label?: string;
+  subgraph?: SerializedGraph;
+}
+
+interface SerializedGraph {
+  id: string;
+  nodes: SerializedNode[];
+  edges: SerializedEdge[];
+}
+
+function serializeGraph(graphId: string, graphsById: Map<string, GraphModel>): SerializedGraph | null {
+  const graph = graphsById.get(graphId);
+  if (!graph) return null;
+
+  return {
+    id: graphId,
+    nodes: graph.nodes.map(node => {
+      const serialized: SerializedNode = { id: node.id, label: node.label };
+      if (graphsById.has(node.id)) {
+        serialized.subgraph = serializeGraph(node.id, graphsById) ?? undefined;
+      }
+      return serialized;
+    }),
+    edges: graph.edges.map(edge => ({ id: edge.id, from: edge.from, to: edge.to })),
+  };
+}
+
+export function saveConceptSketch(): void {
+  const state = useGraphsStore.getState() as { graphsById: Map<string, GraphModel> };
+  const data = serializeGraph('main', state.graphsById);
+
+  const json = JSON.stringify(data, null, 2);
+  const blob = new Blob([json], { type: 'application/json' });
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = 'concept-sketch.json';
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}