diff --git a/src/modules/client/mod.d.ts b/src/modules/client/mod.d.ts
new file mode 100644
index 0000000..3f7713d
--- /dev/null
+++ b/src/modules/client/mod.d.ts
@@ -0,0 +1,6 @@
+import { HotModuleState } from '../modules/client/mod.ts'
+declare global {
+  interface ImportMeta {
+    hot?: HotModuleState
+  }
+}
diff --git a/src/modules/client/mod.ts b/src/modules/client/mod.ts
new file mode 100644
index 0000000..061f93d
--- /dev/null
+++ b/src/modules/client/mod.ts
@@ -0,0 +1,197 @@
+/**
+ * A client-side implementation of the ESM-HMR spec, for real.
+ * See https://github.com/FredKSchott/esm-hmr
+ */
+
+type DisposeCallback = () => void;
+// TODO better typing
+// deno-lint-ignore no-explicit-any
+type AcceptCallback = (args: { module: any; deps: any[] }) => void;
+type AcceptCallbackObject = {
+  deps: string[];
+  callback: AcceptCallback;
+};
+
+// deno-lint-ignore no-explicit-any
+function debug(...args: any[]) {
+  console.log(`[ESM-HMR]`, ...args);
+}
+function reload() {
+  location.reload();
+}
+
+// deno-lint-ignore no-explicit-any
+let SOCKET_MESSAGE_QUEUE: any[] = [];
+// deno-lint-ignore no-explicit-any
+function _sendSocketMessage(socket: WebSocket, msg: any) {
+  socket.send(JSON.stringify(msg));
+}
+// deno-lint-ignore no-explicit-any
+function sendSocketMessage(socket: WebSocket, msg: any) {
+  if (socket.readyState !== socket.OPEN) {
+    SOCKET_MESSAGE_QUEUE.push(msg);
+  } else {
+    _sendSocketMessage(socket, msg);
+  }
+}
+
+const socketURL =
+  // deno-lint-ignore no-explicit-any
+  (window as any).HMR_WEBSOCKET_URL ||
+  // TODO make common function
+  (location.protocol === "http:" ? "ws://" : "wss://") + location.host + "/";
+
+const REGISTERED_MODULES: { [key: string]: HotModuleState } = {};
+
+export class HotModuleState {
+  id: string;
+  #socket: WebSocket;
+  // deno-lint-ignore no-explicit-any
+  data: any = {};
+  isLocked = false;
+  isDeclined = false;
+  isAccepted = false;
+  acceptCallbacks: AcceptCallbackObject[] = [];
+  disposeCallbacks: DisposeCallback[] = [];
+
+  constructor(id: string, socket: WebSocket) {
+    this.id = id;
+    this.#socket = socket;
+  }
+
+  lock(): void {
+    this.isLocked = true;
+  }
+
+  dispose(callback: DisposeCallback): void {
+    this.disposeCallbacks.push(callback);
+  }
+
+  invalidate(): void {
+    reload();
+  }
+
+  decline(): void {
+    this.isDeclined = true;
+  }
+
+  accept(_deps: string[], callback: true | AcceptCallback = true): void {
+    if (this.isLocked) {
+      return;
+    }
+    if (!this.isAccepted) {
+      sendSocketMessage(this.#socket, { id: this.id, type: "hotAccept" });
+      this.isAccepted = true;
+    }
+    if (!Array.isArray(_deps)) {
+      callback = _deps || callback;
+      _deps = [];
+    }
+    if (callback === true) {
+      callback = () => {};
+    }
+    const deps = _deps.map((dep) => {
+      return new URL(dep, `${window.location.origin}${this.id}`).pathname;
+    });
+    this.acceptCallbacks.push({
+      deps,
+      callback,
+    });
+  }
+}
+
+function createHotContext(fullUrl: string, socket: WebSocket) {
+  const id = new URL(fullUrl).pathname;
+  const existing = REGISTERED_MODULES[id];
+  if (existing) {
+    existing.lock();
+    return existing;
+  }
+  const state = new HotModuleState(id, socket);
+  REGISTERED_MODULES[id] = state;
+  return state;
+}
+
+function installHotContext(importMeta: ImportMeta, socket: WebSocket) {
+  // TODO conditionally inject this in build
+  // this condition is a temporary workaround until I figure out how to inject config/env vars,
+  // or accomplish the above TODO
+  if (location.hostname === "localhost") {
+    importMeta.hot = createHotContext(importMeta.url, socket);
+  }
+}
+
+async function applyUpdate(id: string) {
+  const state = REGISTERED_MODULES[id];
+  if (!state) {
+    return false;
+  }
+  if (state.isDeclined) {
+    return false;
+  }
+
+  const acceptCallbacks = state.acceptCallbacks;
+  const disposeCallbacks = state.disposeCallbacks;
+  state.disposeCallbacks = [];
+  state.data = {};
+
+  disposeCallbacks.map((callback) => callback());
+  const updateID = Date.now();
+  for (const { deps, callback: acceptCallback } of acceptCallbacks) {
+    const [module, ...depModules] = await Promise.all([
+      import(id + `?mtime=${updateID}`),
+      ...deps.map((d) => import(d + `?mtime=${updateID}`)),
+    ]);
+    acceptCallback({ module, deps: depModules });
+  }
+
+  return true;
+}
+
+let isHmrClientRunning = false
+function startHmrClient(socket: WebSocket) {
+  socket.addEventListener("open", () => {
+    SOCKET_MESSAGE_QUEUE.forEach((msg) => _sendSocketMessage(socket, msg));
+    SOCKET_MESSAGE_QUEUE = [];
+  });
+  socket.addEventListener("message", async ({ data: _data }) => {
+    if (!_data) {
+      return;
+    }
+    const data = JSON.parse(_data);
+    debug("message", data);
+    if (data.type === "reload") {
+      debug("message: reload");
+      reload();
+      return;
+    }
+    if (data.type !== "update") {
+      debug("message: unknown", data);
+      return;
+    }
+    debug("message: update", data);
+    debug(data.url, Object.keys(REGISTERED_MODULES));
+    try {
+      const ok = await applyUpdate(data.url)
+      if (!ok) {
+        reload();
+      }
+    } catch (err) {
+      console.error(err);
+      reload();
+    }
+  });
+
+  isHmrClientRunning = true
+  debug("listening for file changes...");
+}
+
+export function useHmr(importMeta: ImportMeta) {
+  if(!isHmrClientRunning) {
+    // Seems like Deno cannot handle subprotocols
+    // const socket = new WebSocket(socketURL, "esm-hmr");
+    const socket = new WebSocket(socketURL);
+    startHmrClient(socket)
+    installHotContext(importMeta, socket)
+  }
+}
diff --git a/src/modules/server/mod.ts b/src/modules/server/mod.ts
new file mode 100644
index 0000000..f63291b
--- /dev/null
+++ b/src/modules/server/mod.ts
@@ -0,0 +1,147 @@
+/**
+ * A server-side implementation of the ESM-HMR spec, for real.
+ * See https://github.com/FredKSchott/esm-hmr
+ *
+ * TODO there's some confusing naming conventions going on in this code
+ * The names "id" and "url" are used to refer to the same things, and those
+ * things are actually both path(name)s.
+ */
+
+interface Dependency {
+  dependents: Set<string>;
+  dependencies: Set<string>;
+  isHmrEnabled: boolean;
+  isHmrAccepted: boolean;
+  needsReplacement: boolean;
+}
+
+interface ModuleEventSource {
+  (emitModuleModifiedEvent: (moduleId: string) => void): void
+}
+
+export class EsmHmrEngine {
+  clients: Set<WebSocket> = new Set();
+  dependencyTree = new Map<string, Dependency>();
+
+  constructor(watcher: ModuleEventSource) {
+    watcher((url) => {
+      this.broadcastMessage({ type: "update", url });
+    })
+  }
+
+  addClient(client: WebSocket): void {
+    const onopen = () => {
+      this.connectClient(client);
+      this.registerListener(client);
+    };
+    client.onopen = onopen;
+    if (client.readyState === WebSocket.OPEN) {
+      onopen();
+    }
+    client.onerror = () => {
+      console.warn(`Socket error!`);
+    };
+    client.onclose = (e) => {
+      console.info("socket closed", e.reason, e.code, e.wasClean);
+      this.clients.delete(client);
+    };
+  }
+
+  registerListener(client: WebSocket) {
+    client.onmessage = (event) => {
+      console.info("received message:", event.data);
+      const message = JSON.parse(event.data.toString());
+      if (message.type === "hotAccept") {
+        const entry = this.getEntry(message.id, true) as Dependency;
+        entry.isHmrAccepted = true;
+      }
+    };
+  }
+
+  createEntry(sourceUrl: string) {
+    const newEntry: Dependency = {
+      dependencies: new Set(),
+      dependents: new Set(),
+      needsReplacement: false,
+      isHmrEnabled: false,
+      isHmrAccepted: false,
+    };
+    this.dependencyTree.set(sourceUrl, newEntry);
+    return newEntry;
+  }
+
+  getEntry(sourceUrl: string, createIfNotFound = false) {
+    const result = this.dependencyTree.get(sourceUrl);
+    if (result) {
+      return result;
+    }
+    if (createIfNotFound) {
+      return this.createEntry(sourceUrl);
+    }
+    return null;
+  }
+
+  setEntry(sourceUrl: string, imports: string[], isHmrEnabled = false) {
+    const result = this.getEntry(sourceUrl, true)!;
+    const outdatedDependencies = new Set(result.dependencies);
+    result.isHmrEnabled = isHmrEnabled;
+    for (const importUrl of imports) {
+      this.addRelationship(sourceUrl, importUrl);
+      outdatedDependencies.delete(importUrl);
+    }
+    for (const importUrl of outdatedDependencies) {
+      this.removeRelationship(sourceUrl, importUrl);
+    }
+  }
+
+  removeRelationship(sourceUrl: string, importUrl: string) {
+    const importResult = this.getEntry(importUrl);
+    importResult && importResult.dependents.delete(sourceUrl);
+    const sourceResult = this.getEntry(sourceUrl);
+    sourceResult && sourceResult.dependencies.delete(importUrl);
+  }
+
+  addRelationship(sourceUrl: string, importUrl: string) {
+    if (importUrl !== sourceUrl) {
+      const importResult = this.getEntry(importUrl, true)!;
+      importResult.dependents.add(sourceUrl);
+      const sourceResult = this.getEntry(sourceUrl, true)!;
+      sourceResult.dependencies.add(importUrl);
+    }
+  }
+
+  markEntryForReplacement(entry: Dependency, state: boolean) {
+    entry.needsReplacement = state;
+  }
+
+  broadcastMessage(data: Record<string, unknown>) {
+    console.log("broadcast:", data);
+    this.clients.forEach((client) => {
+      if (client.readyState === WebSocket.OPEN) {
+        client.send(JSON.stringify(data));
+      } else {
+        this.disconnectClient(client);
+      }
+    });
+  }
+
+  connectClient(client: WebSocket) {
+    console.info("client connected");
+    this.clients.add(client);
+  }
+
+  disconnectClient(client: WebSocket) {
+    console.log("disconnecting client")
+    client.close();
+    this.clients.delete(client);
+  }
+
+  disconnectAllClients() {
+    for (const client of this.clients) {
+      this.disconnectClient(client);
+    }
+  }
+}
+
+
+