diff --git a/src/app.test.tsx b/src/app.test.tsx
index 871d930..17b6cf3 100644
--- a/src/app.test.tsx
+++ b/src/app.test.tsx
@@ -79,10 +79,10 @@ describe("app", () => {
     expect(await screen.findByText(/GET \/readyz/)).toBeInTheDocument();
     expect(screen.getByText(/"status": "ready"/)).toBeInTheDocument();
     expect(screen.getByText(/"service": "svc"/)).toBeInTheDocument();
-    expect(fetchMock.mock.calls.map((c) => c[0])).toEqual([
-      "/api/",
-      "/api/readyz",
-    ]);
+    // Layout also asks /auth/me who is signed in; that's not what's under test.
+    expect(
+      fetchMock.mock.calls.map((c) => c[0]).filter((u) => u !== "/api/auth/me"),
+    ).toEqual(["/api/", "/api/readyz"]);
   });
 
   it("status shows the error page when the backend is unreachable", async () => {
diff --git a/src/lib/auth.ts b/src/lib/auth.ts
new file mode 100644
index 0000000..9053377
--- /dev/null
+++ b/src/lib/auth.ts
@@ -0,0 +1,45 @@
+import { redirect } from "react-router";
+import { ApiError, call } from "./stories";
+
+export interface User {
+  id: string;
+  username: string;
+}
+
+export const fetchMe = () =>
+  call<{ user: User | null }>("GET", "/auth/me").then((r) => r.user);
+export const login = (username: string, password: string) =>
+  call<{ user: User }>("POST", "/auth/login", { username, password }).then(
+    (r) => r.user,
+  );
+export const register = (username: string, password: string) =>
+  call<{ user: User }>("POST", "/auth/register", { username, password }).then(
+    (r) => r.user,
+  );
+export const logout = () => call<void>("POST", "/auth/logout");
+
+/** Root loader: who am I? A failure just means "not signed in". */
+export const authLoader = () => fetchMe().catch(() => null);
+
+/** Run a route loader; a 401 sends the visitor to /login and back again. */
+export async function withLogin<T>(
+  request: Request,
+  load: () => Promise<T>,
+): Promise<T> {
+  try {
+    return await load();
+  } catch (e) {
+    if (e instanceof ApiError && e.status === 401) {
+      const url = new URL(request.url);
+      throw redirect(
+        `/login?next=${encodeURIComponent(url.pathname + url.search)}`,
+      );
+    }
+    throw e;
+  }
+}
+
+/** Only same-site paths are valid post-login destinations (no open redirect). */
+export function safeNext(next: string | null): string {
+  return next && /^\/(?![/\\])/.test(next) ? next : "/build";
+}
diff --git a/src/lib/stories.ts b/src/lib/stories.ts
index 06b990a..86a02c8 100644
--- a/src/lib/stories.ts
+++ b/src/lib/stories.ts
@@ -47,7 +47,7 @@ export class ApiError extends Error {
   }
 }
 
-async function call<T>(
+export async function call<T>(
   method: string,
   path: string,
   body?: unknown,
@@ -85,7 +85,10 @@ export const listStories = (mine: boolean) =>
     `/stories${mine ? "?mine=1" : ""}`,
   ).then((r) => r.stories);
 export const getStory = (id: string) =>
-  call<{ story: Story; issues: Issue[] }>("GET", `/stories/${id}`);
+  call<{ story: Story; issues: Issue[]; ownedByMe: boolean }>(
+    "GET",
+    `/stories/${id}`,
+  );
 export const createStory = (input: StoryInput) =>
   call<{ story: Story; issues: Issue[] }>("POST", "/stories", input);
 export const saveStory = (id: string, input: StoryInput) =>
diff --git a/src/routes.tsx b/src/routes.tsx
index 2006760..96bcf29 100644
--- a/src/routes.tsx
+++ b/src/routes.tsx
@@ -1,9 +1,11 @@
 import type { RouteObject } from "react-router";
+import { authLoader, withLogin } from "./lib/auth";
 import { Build, buildLoader } from "./routes/Build";
 import { BuildList, buildListLoader } from "./routes/BuildList";
 import { ErrorPage } from "./routes/ErrorPage";
 import { Layout } from "./routes/Layout";
 import { Library, libraryLoader } from "./routes/Library";
+import { Login } from "./routes/Login";
 import { Play, playLoader } from "./routes/Play";
 import { Status, statusLoader } from "./routes/Status";
 
@@ -12,12 +14,22 @@ export const routes: RouteObject[] = [
   {
     path: "/",
     element: <Layout />,
+    loader: authLoader,
     errorElement: <ErrorPage />,
     children: [
       { index: true, element: <Library />, loader: libraryLoader },
       { path: "play/:id", element: <Play />, loader: playLoader },
-      { path: "build", element: <BuildList />, loader: buildListLoader },
-      { path: "build/:id", element: <Build />, loader: buildLoader },
+      { path: "login", element: <Login /> },
+      {
+        path: "build",
+        element: <BuildList />,
+        loader: ({ request }) => withLogin(request, buildListLoader),
+      },
+      {
+        path: "build/:id",
+        element: <Build />,
+        loader: (args) => withLogin(args.request, () => buildLoader(args)),
+      },
       { path: "status", element: <Status />, loader: statusLoader },
     ],
   },
diff --git a/src/routes/Build.tsx b/src/routes/Build.tsx
index 4e2b84e..62f8861 100644
--- a/src/routes/Build.tsx
+++ b/src/routes/Build.tsx
@@ -1,5 +1,10 @@
 import { useMemo, useState } from "react";
-import { Link, useLoaderData, type LoaderFunctionArgs } from "react-router";
+import {
+  Link,
+  redirect,
+  useLoaderData,
+  type LoaderFunctionArgs,
+} from "react-router";
 import { downloadJson, exportStory } from "../lib/exchange";
 import {
   analyze,
@@ -20,7 +25,11 @@ import {
 } from "../lib/stories";
 
 export async function buildLoader({ params }: LoaderFunctionArgs) {
-  return (await getStory(params.id!)).story;
+  const { story, ownedByMe } = await getStory(params.id!);
+  // Someone else's published story: send them to the player, not an editor
+  // whose saves would 403.
+  if (!ownedByMe) throw redirect(`/play/${story.id}`);
+  return story;
 }
 
 export function Build() {
diff --git a/src/routes/Layout.tsx b/src/routes/Layout.tsx
index ab27d83..3ab33ca 100644
--- a/src/routes/Layout.tsx
+++ b/src/routes/Layout.tsx
@@ -1,7 +1,25 @@
-import { NavLink, Outlet } from "react-router";
+import {
+  Link,
+  NavLink,
+  Outlet,
+  useLoaderData,
+  useNavigate,
+  useRevalidator,
+} from "react-router";
+import { logout, type User } from "../lib/auth";
 import { getConfig } from "../lib/config";
 
 export function Layout() {
+  const user = useLoaderData() as User | null;
+  const navigate = useNavigate();
+  const { revalidate } = useRevalidator();
+
+  async function signOut() {
+    await logout().catch(() => {});
+    navigate("/");
+    await revalidate();
+  }
+
   return (
     <>
       <header>
@@ -13,6 +31,18 @@ export function Layout() {
           <NavLink to="/build">Build</NavLink>
           <NavLink to="/status">Status</NavLink>
         </nav>
+        <span className="who">
+          {user ? (
+            <>
+              <span data-testid="whoami">{user.username}</span>{" "}
+              <button className="link" onClick={() => void signOut()}>
+                Log out
+              </button>
+            </>
+          ) : (
+            <Link to="/login">Log in</Link>
+          )}
+        </span>
       </header>
       <main>
         <Outlet />
diff --git a/src/routes/Login.tsx b/src/routes/Login.tsx
new file mode 100644
index 0000000..2761455
--- /dev/null
+++ b/src/routes/Login.tsx
@@ -0,0 +1,72 @@
+import { useState, type FormEvent } from "react";
+import { useNavigate, useRevalidator, useSearchParams } from "react-router";
+import { login, register, safeNext } from "../lib/auth";
+
+export function Login() {
+  const [params] = useSearchParams();
+  const navigate = useNavigate();
+  const { revalidate } = useRevalidator();
+  const [mode, setMode] = useState<"login" | "register">("login");
+  const [username, setUsername] = useState("");
+  const [password, setPassword] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [busy, setBusy] = useState(false);
+
+  async function submit(e: FormEvent) {
+    e.preventDefault();
+    setBusy(true);
+    setError(null);
+    try {
+      await (mode === "login" ? login : register)(username, password);
+      await revalidate();
+      navigate(safeNext(params.get("next")), { replace: true });
+    } catch (err) {
+      setError(err instanceof Error ? err.message : "Something went wrong");
+    } finally {
+      setBusy(false);
+    }
+  }
+
+  const isLogin = mode === "login";
+  return (
+    <section className="editor auth">
+      <h1>{isLogin ? "Log in" : "Create an account"}</h1>
+      <form onSubmit={submit}>
+        <label>
+          Username
+          <input
+            value={username}
+            autoComplete="username"
+            required
+            onChange={(e) => setUsername(e.target.value)}
+          />
+        </label>
+        <label>
+          Password
+          <input
+            type="password"
+            value={password}
+            autoComplete={isLogin ? "current-password" : "new-password"}
+            required
+            minLength={isLogin ? undefined : 8}
+            onChange={(e) => setPassword(e.target.value)}
+          />
+        </label>
+        {error && <p role="alert">{error}</p>}
+        <button type="submit" disabled={busy}>
+          {isLogin ? "Log in" : "Sign up"}
+        </button>{" "}
+        <button
+          type="button"
+          className="link"
+          onClick={() => {
+            setMode(isLogin ? "register" : "login");
+            setError(null);
+          }}
+        >
+          {isLogin ? "Need an account? Sign up" : "Have an account? Log in"}
+        </button>
+      </form>
+    </section>
+  );
+}
diff --git a/src/routes/auth.test.tsx b/src/routes/auth.test.tsx
new file mode 100644
index 0000000..5fbd5f2
--- /dev/null
+++ b/src/routes/auth.test.tsx
@@ -0,0 +1,201 @@
+import { render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { createMemoryRouter, RouterProvider } from "react-router";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { safeNext } from "../lib/auth";
+import { routes } from "../routes";
+
+vi.mock("@unleash/proxy-client-react", () => ({
+  FlagProvider: ({ children }: { children: React.ReactNode }) => children,
+  useFlag: () => false,
+  useVariant: () => ({ name: "disabled", enabled: false }),
+}));
+
+type Handler = (init?: RequestInit) => Response | Promise<Response>;
+const json = (body: unknown, status = 200) =>
+  new Response(JSON.stringify(body), { status });
+
+/** fetch stub dispatching on "METHOD /path"; unknown calls fail the test loudly. */
+function mockApi(handlers: Record<string, Handler>) {
+  const calls: string[] = [];
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async (url: string, init?: RequestInit) => {
+      const key = `${init?.method ?? "GET"} ${url.replace(/^\/api/, "")}`;
+      calls.push(key);
+      const h = handlers[key];
+      if (!h) throw new Error(`unexpected request: ${key}`);
+      return h(init);
+    }),
+  );
+  return calls;
+}
+
+function renderAt(path: string) {
+  const router = createMemoryRouter(routes, { initialEntries: [path] });
+  render(<RouterProvider router={router} />);
+  return router;
+}
+
+beforeEach(() => {
+  window.__APP_CONFIG__ = { API_BASE_URL: "/api", APP_NAME: "t" };
+});
+
+describe("auth UI", () => {
+  it("shows a Log in link when signed out and the username when signed in", async () => {
+    mockApi({
+      "GET /auth/me": () => json({ user: null }),
+      "GET /stories": () => json({ stories: [] }),
+    });
+    renderAt("/");
+    expect(
+      await screen.findByRole("link", { name: "Log in" }),
+    ).toBeInTheDocument();
+
+    mockApi({
+      "GET /auth/me": () => json({ user: { id: "1", username: "alice" } }),
+      "GET /stories": () => json({ stories: [] }),
+    });
+    renderAt("/");
+    expect(await screen.findByTestId("whoami")).toHaveTextContent("alice");
+  });
+
+  it("redirects /build to /login with next= when the API says 401", async () => {
+    mockApi({
+      "GET /auth/me": () => json({ user: null }),
+      "GET /stories?mine=1": () =>
+        json({ error: "authentication required" }, 401),
+    });
+    const router = renderAt("/build");
+    expect(
+      await screen.findByRole("heading", { name: "Log in" }),
+    ).toBeInTheDocument();
+    expect(router.state.location.pathname).toBe("/login");
+    expect(router.state.location.search).toBe("?next=%2Fbuild");
+  });
+
+  it("logs in, returns to next, and shows server errors for bad credentials", async () => {
+    const user = userEvent.setup();
+    let signedIn = false;
+    mockApi({
+      "GET /auth/me": () =>
+        json({ user: signedIn ? { id: "1", username: "alice" } : null }),
+      "GET /stories?mine=1": () => json({ stories: [] }),
+      "POST /auth/login": (init) => {
+        const body = JSON.parse(String(init?.body));
+        if (body.password !== "correct horse") {
+          return json({ error: "invalid username or password" }, 401);
+        }
+        signedIn = true;
+        return json({ user: { id: "1", username: "alice" } });
+      },
+    });
+    const router = renderAt("/login?next=%2Fbuild");
+    await user.type(await screen.findByLabelText("Username"), "alice");
+    await user.type(screen.getByLabelText("Password"), "wrong");
+    await user.click(screen.getByRole("button", { name: "Log in" }));
+    expect(await screen.findByRole("alert")).toHaveTextContent(
+      "invalid username or password",
+    );
+
+    await user.clear(screen.getByLabelText("Password"));
+    await user.type(screen.getByLabelText("Password"), "correct horse");
+    await user.click(screen.getByRole("button", { name: "Log in" }));
+    expect(
+      await screen.findByRole("heading", { name: "Your stories" }),
+    ).toBeInTheDocument();
+    expect(router.state.location.pathname).toBe("/build");
+    expect(screen.getByTestId("whoami")).toHaveTextContent("alice");
+  });
+
+  it("registers a new account", async () => {
+    const user = userEvent.setup();
+    const calls = mockApi({
+      "GET /auth/me": () => json({ user: null }),
+      "GET /stories?mine=1": () => json({ stories: [] }),
+      "POST /auth/register": (init) => {
+        expect(JSON.parse(String(init?.body))).toEqual({
+          username: "bob",
+          password: "correct horse",
+        });
+        return json({ user: { id: "2", username: "bob" } }, 201);
+      },
+    });
+    renderAt("/login");
+    await user.click(await screen.findByRole("button", { name: /Sign up/ }));
+    expect(
+      screen.getByRole("heading", { name: "Create an account" }),
+    ).toBeInTheDocument();
+    await user.type(screen.getByLabelText("Username"), "bob");
+    await user.type(screen.getByLabelText("Password"), "correct horse");
+    await user.click(screen.getByRole("button", { name: "Sign up" }));
+    await waitFor(() => expect(calls).toContain("POST /auth/register"));
+    expect(
+      await screen.findByRole("heading", { name: "Your stories" }),
+    ).toBeInTheDocument();
+  });
+
+  it("logs out and returns to the library", async () => {
+    const user = userEvent.setup();
+    let signedIn = true;
+    mockApi({
+      "GET /auth/me": () =>
+        json({ user: signedIn ? { id: "1", username: "alice" } : null }),
+      "GET /stories": () => json({ stories: [] }),
+      "POST /auth/logout": () => {
+        signedIn = false;
+        return new Response(null, { status: 204 });
+      },
+    });
+    renderAt("/");
+    await user.click(await screen.findByRole("button", { name: "Log out" }));
+    expect(
+      await screen.findByRole("link", { name: "Log in" }),
+    ).toBeInTheDocument();
+    expect(screen.queryByTestId("whoami")).not.toBeInTheDocument();
+  });
+
+  it("sends editors of someone else's published story to the player", async () => {
+    mockApi({
+      "GET /auth/me": () => json({ user: { id: "2", username: "bob" } }),
+      "GET /stories/s1": () =>
+        json({
+          story: {
+            id: "s1",
+            title: "Cave",
+            description: "",
+            published: true,
+            updatedAt: "",
+            doc: {
+              start: "a",
+              nodes: {
+                a: { title: "A", text: "hi", ending: "win", choices: [] },
+              },
+            },
+          },
+          issues: [],
+          ownedByMe: false,
+        }),
+    });
+    const router = renderAt("/build/s1");
+    expect(await screen.findByText("hi")).toBeInTheDocument();
+    expect(router.state.location.pathname).toBe("/play/s1");
+  });
+});
+
+describe("safeNext", () => {
+  it("allows same-site paths and rejects anything that could leave the site", () => {
+    expect(safeNext("/build/abc?x=1")).toBe("/build/abc?x=1");
+    for (const bad of [
+      null,
+      "",
+      "build",
+      "//evil.com",
+      "/\\evil.com",
+      "https://evil.com",
+      "javascript:alert(1)",
+    ]) {
+      expect(safeNext(bad)).toBe("/build");
+    }
+  });
+});
diff --git a/src/styles.css b/src/styles.css
index 78490ca..ff1ec70 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -164,3 +164,15 @@ button.danger {
 .choice-row select {
   margin-top: 0 !important;
 }
+header .who {
+  margin-left: auto;
+}
+header .who button.link {
+  color: #f5e9d3;
+}
+.auth {
+  max-width: 24rem;
+}
+[role="alert"] {
+  color: #a31d1d;
+}