diff --git a/components/app.js b/components/app.js
index 46b9cdf..5615824 100644
--- a/components/app.js
+++ b/components/app.js
@@ -577,6 +577,7 @@ export default class App extends Component {
 				}
 				this.createBuffer(serverID, buf.name);
 				client.who(buf.name);
+				client.monitor(buf.name);
 			}
 
 			let lastReceipt = this.latestReceipt(ReceiptType.DELIVERED);
@@ -785,6 +786,8 @@ export default class App extends Component {
 		case irc.RPL_TOPICWHOTIME:
 		case irc.RPL_NAMREPLY:
 		case irc.RPL_ENDOFNAMES:
+		case irc.RPL_MONONLINE:
+		case irc.RPL_MONOFFLINE:
 		case "AWAY":
 		case "SETNAME":
 		case "CAP":
@@ -853,6 +856,7 @@ export default class App extends Component {
 			client.send({ command: "JOIN", params: [target] });
 		} else {
 			client.who(target);
+			client.monitor(target);
 			this.createBuffer(serverID, target);
 			this.switchBuffer({ server: serverID, name: target });
 		}
@@ -928,6 +932,8 @@ export default class App extends Component {
 				return { buffers };
 			});
 
+			client.unmonitor(buf.name);
+
 			this.receipts.delete(buf.name);
 			this.saveReceipts();
 
diff --git a/lib/client.js b/lib/client.js
index 1622479..6bf4cbd 100644
--- a/lib/client.js
+++ b/lib/client.js
@@ -54,6 +54,7 @@ export default class Client extends EventTarget {
 	pingIntervalID = null;
 	pendingHistory = Promise.resolve(null);
 	cm = irc.CaseMapping.RFC1459;
+	monitored = new irc.CaseMapMap(null, irc.CaseMapping.RFC1459);
 	whoisDB = new irc.CaseMapMap(null, irc.CaseMapping.RFC1459);
 
 	constructor(params) {
@@ -97,6 +98,7 @@ export default class Client extends EventTarget {
 			this.batches = new Map();
 			this.pendingHistory = Promise.resolve(null);
 			this.isupport = new Map();
+			this.monitored = new irc.CaseMapMap(null, irc.CaseMapping.RFC1459);
 
 			if (this.autoReconnect) {
 				if (!navigator.onLine) {
@@ -199,6 +201,10 @@ export default class Client extends EventTarget {
 			if (changed.indexOf("CASEMAPPING") >= 0) {
 				this.setCaseMapping(this.isupport.get("CASEMAPPING"));
 			}
+			if (changed.indexOf("MONITOR") >= 0 && this.isupport.has("MONITOR")) {
+				let targets = Array.from(this.monitored.keys()).slice(0, this.maxMonitorTargets());
+				this.send({ command: "MONITOR", params: ["+", targets.join(",")] });
+			}
 			break;
 		case irc.RPL_ENDOFMOTD:
 		case irc.ERR_NOMOTD:
@@ -478,6 +484,7 @@ export default class Client extends EventTarget {
 		}
 
 		this.whoisDB = new irc.CaseMapMap(this.whoisDB, this.cm);
+		this.monitored = new irc.CaseMapMap(this.monitored, this.cm);
 	}
 
 	isServer(name) {
@@ -683,4 +690,40 @@ export default class Client extends EventTarget {
 			return networks;
 		});
 	}
+
+	maxMonitorTargets() {
+		if (!this.isupport.has("MONITOR")) {
+			return 0;
+		}
+		return parseInt(this.isupport.get("MONITOR"), 10);
+	}
+
+	monitor(target) {
+		if (this.monitored.has(target)) {
+			return;
+		}
+
+		this.monitored.set(target, true);
+
+		// TODO: add poll-based fallback when MONITOR is not supported
+		if (this.monitored.size + 1 > this.maxMonitorTargets()) {
+			return;
+		}
+
+		this.send({ command: "MONITOR", params: ["+", target] });
+	}
+
+	unmonitor(target) {
+		if (!this.monitored.has(target)) {
+			return;
+		}
+
+		this.monitored.delete(target);
+
+		if (!this.isupport.has("MONITOR")) {
+			return;
+		}
+
+		this.send({ command: "MONITOR", params: ["-", target] });
+	}
 }
diff --git a/lib/irc.js b/lib/irc.js
index a45f970..5024e7d 100644
--- a/lib/irc.js
+++ b/lib/irc.js
@@ -42,6 +42,12 @@ export const ERR_UNAVAILRESOURCE = "437";
 // Other
 export const RPL_QUIETLIST = "728";
 export const RPL_ENDOFQUIETLIST = "729";
+// IRCv3 MONITOR: https://ircv3.net/specs/extensions/monitor
+export const RPL_MONONLINE = "730";
+export const RPL_MONOFFLINE = "731";
+export const RPL_MONLIST = "732";
+export const RPL_ENDOFMONLIST = "733";
+export const ERR_MONLISTFULL = "734";
 // IRCv3 SASL: https://ircv3.net/specs/extensions/sasl-3.1
 export const RPL_LOGGEDIN = "900";
 export const RPL_LOGGEDOUT = "901";
@@ -107,7 +113,7 @@ export function formatTags(tags) {
 	return l.join(";");
 }
 
-function parsePrefix(s) {
+export function parsePrefix(s) {
 	let prefix = {
 		name: null,
 		user: null,
@@ -306,6 +312,7 @@ export function isError(cmd) {
 	case ERR_SASLTOOLONG:
 	case ERR_SASLABORTED:
 	case ERR_SASLALREADY:
+	case ERR_MONLISTFULL:
 		return true;
 	case "FAIL":
 		return true;
diff --git a/state.js b/state.js
index c0f70d4..ab7ccbe 100644
--- a/state.js
+++ b/state.js
@@ -293,7 +293,7 @@ export const State = {
 			return;
 		}
 
-		let target, channel, topic;
+		let target, channel, topic, targets;
 		switch (msg.command) {
 		case irc.RPL_MYINFO:
 			// TODO: parse available modes
@@ -437,6 +437,30 @@ export const State = {
 
 				return { members };
 			});
+		case irc.RPL_MONONLINE:
+			targets = msg.params[1].split(",");
+
+			for (let target of targets) {
+				let prefix = irc.parsePrefix(target);
+				let update = updateBuffer(prefix.name, (buf) => {
+					return { offline: false };
+				});
+				state = { ...state, ...update };
+			}
+
+			return state;
+		case irc.RPL_MONOFFLINE:
+			targets = msg.params[1].split(",");
+
+			for (let target of targets) {
+				let prefix = irc.parsePrefix(target);
+				let update = updateBuffer(prefix.name, (buf) => {
+					return { offline: true };
+				});
+				state = { ...state, ...update };
+			}
+
+			return state;
 		}
 	},
 	addMessage(state, msg, bufID) {