diff --git a/http2irc.py b/http2irc.py
index a3e7ef0..f36d179 100644
--- a/http2irc.py
+++ b/http2irc.py
@@ -6,6 +6,7 @@ import collections
 import concurrent.futures
 import importlib.util
 import inspect
+import itertools
 import logging
 import os.path
 import signal
@@ -84,8 +85,12 @@ class Config(dict):
 				raise InvalidConfig(f'Invalid IRC SSL setting: {obj["irc"]["ssl"]!r}')
 			if 'nick' in obj['irc'] and not isinstance(obj['irc']['nick'], str): #TODO: Check whether it's a valid nickname
 				raise InvalidConfig('Invalid IRC nick')
+			if len(IRCClientProtocol.nick_command(obj['irc']['nick'])) > 510:
+				raise InvalidConfig('Invalid IRC nick: NICK command too long')
 			if 'real' in obj['irc'] and not isinstance(obj['irc']['real'], str):
 				raise InvalidConfig('Invalid IRC realname')
+			if len(IRCClientProtocol.user_command(obj['irc']['nick'], obj['irc']['real'])) > 510:
+				raise InvalidConfig('Invalid IRC nick/realname combination: USER command too long')
 			if ('certfile' in obj['irc']) != ('certkeyfile' in obj['irc']):
 				raise InvalidConfig('Invalid IRC cert config: needs both certfile and certkeyfile')
 			if 'certfile' in obj['irc']:
@@ -137,6 +142,8 @@ class Config(dict):
 					raise InvalidConfig(f'Invalid map {key!r} IRC channel: does not start with # or &')
 				if any(x in map_['ircchannel'][1:] for x in (' ', '\x00', '\x07', '\r', '\n', ',')):
 					raise InvalidConfig(f'Invalid map {key!r} IRC channel: contains forbidden characters')
+				if 14 + len(map_['ircchannel']) > 510: # 14 = prefix 'PRIVMSG ' + suffix ' :' + at least one UTF-8 character; implicitly also covers the shorter JOIN/PART messages
+					raise InvalidConfig(f'Invalid map {key!r} IRC channel: too long')
 
 				if 'auth' in map_:
 					if map_['auth'] is not False and not isinstance(map_['auth'], str):
@@ -282,15 +289,56 @@ class IRCClientProtocol(asyncio.Protocol):
 		self.sasl = bool(self.config['irc']['certfile'] and self.config['irc']['certkeyfile'])
 		self.authenticated = False
 
+	@staticmethod
+	def nick_command(nick: str):
+		return b'NICK ' + nick.encode('utf-8')
+
+	@staticmethod
+	def user_command(nick: str, real: str):
+		nickb = nick.encode('utf-8')
+		return b'USER ' + nickb + b' ' + nickb + b' ' + nickb + b' :' + real.encode('utf-8')
+
 	def connection_made(self, transport):
 		self.logger.info('IRC connected')
 		self.transport = transport
 		self.connected = True
-		nickb = self.config['irc']['nick'].encode('utf-8')
 		if self.sasl:
 			self.send(b'CAP REQ :sasl')
-		self.send(b'NICK ' + nickb)
-		self.send(b'USER ' + nickb + b' ' + nickb + b' ' + nickb + b' :' + self.config['irc']['real'].encode('utf-8'))
+		self.send(self.nick_command(self.config['irc']['nick']))
+		self.send(self.user_command(self.config['irc']['nick'], self.config['irc']['real']))
+
+	def _send_join_part(self, command, channels):
+		'''Split a JOIN or PART into multiple messages as necessary'''
+		# command: b'JOIN' or b'PART'; channels: set[str]
+
+		channels = [x.encode('utf-8') for x in channels]
+		if len(command) + sum(1 + len(x) for x in channels) <= 510: # Total length = command + (separator + channel name for each channel, where the separator is a space for the first and then a comma)
+			# Everything fits into one command.
+			self.send(command + b' ' + b','.join(channels))
+			return
+
+		# List too long, need to split.
+		limit = 510 - len(command)
+		lengths = [1 + len(x) for x in channels]  # separator + channel name
+		chanLengthAcceptable = [l <= limit for l in lengths]
+		if not all(chanLengthAcceptable):
+			# There are channel names that are too long to even fit into one message on their own; filter them out and warn about them.
+			# This should never happen since the config reader would already filter it out.
+			tooLongChannels = [x for x, a in zip(channels, chanLengthAcceptable) if not a]
+			channels = [x for x, a in zip(channels, chanLengthAcceptable) if a]
+			lengths = [l for l, a in zip(lengths, chanLengthAcceptable) if a]
+			for channel in tooLongChannels:
+				self.logger.warning(f'Cannot {command} {channel}: name too long')
+		runningLengths = list(itertools.accumulate(lengths))  # entry N = length of all entries up to and including channel N, including separators
+		offset = 0
+		while channels:
+			i = next((x[0] for x in enumerate(runningLengths) if x[1] - offset > limit), -1)
+			if i == -1: # Last batch
+				i = len(channels)
+			self.send(command + b' ' + b','.join(channels[:i]))
+			offset = runningLengths[i-1]
+			channels = channels[i:]
+			runningLengths = runningLengths[i:]
 
 	def update_channels(self, channels: set):
 		channelsToPart = self.channels - channels
@@ -299,10 +347,9 @@ class IRCClientProtocol(asyncio.Protocol):
 
 		if self.connected:
 			if channelsToPart:
-				#TODO: Split if too long
-				self.send(b'PART ' + ','.join(channelsToPart).encode('utf-8'))
+				self._send_join_part(b'PART', channelsToPart)
 			if channelsToJoin:
-				self.send(b'JOIN ' + ','.join(channelsToJoin).encode('utf-8'))
+				self._send_join_part(b'JOIN', channelsToJoin)
 
 	def send(self, data):
 		self.logger.debug(f'Send: {data!r}')
@@ -335,11 +382,47 @@ class IRCClientProtocol(asyncio.Protocol):
 			self.logger.debug(f'Got message: {message!r}')
 			if message is None:
 				break
-			self.logger.info(f'Sending {message!r} to {channel!r}')
-			#TODO Split if the message is too long.
-			self.unconfirmedMessages.append((channel, message))
-			self.send(b'PRIVMSG ' + channel.encode('utf-8') + b' :' + message.encode('utf-8'))
-			await asyncio.sleep(1) # Rate limit
+			channelB = channel.encode('utf-8')
+			messageB = message.encode('utf-8')
+			if len(b'PRIVMSG ' + channelB + b' :' + messageB) > 510:
+				self.logger.debug(f'Splitting up into smaller messages')
+				# Message too long, need to split. First try to split on spaces, then on codepoints. Ideally, would use graphemes between, but that's too complicated.
+				prefix = b'PRIVMSG ' + channelB + b' :'
+				prefixLength = len(prefix)
+				maxMessageLength = 510 - prefixLength # maximum length of the message part within each line
+				messages = []
+				while message:
+					if len(messageB) <= maxMessageLength:
+						messages.append(message)
+						break
+
+					spacePos = messageB.rfind(b' ', 0, maxMessageLength + 1)
+					if spacePos != -1:
+						messages.append(messageB[:spacePos].decode('utf-8'))
+						messageB = messageB[spacePos + 1:]
+						message = messageB.decode('utf-8')
+						continue
+
+					# No space found, need to search for a suitable codepoint location.
+					pMessage = message[:maxMessageLength] # at most 510 codepoints which expand to at least 510 bytes
+					pLengths = [len(x.encode('utf-8')) for x in pMessage] # byte size of each codepoint
+					pRunningLengths = list(itertools.accumulate(pLengths)) # byte size up to each codepoint
+					if pRunningLengths[-1] <= maxMessageLength: # Special case: entire pMessage is short enough
+						messages.append(pMessage)
+						message = message[maxMessageLength:]
+						messageB = message.encode('utf-8')
+						continue
+					cutoffIndex = next(x[0] for x in enumerate(pRunningLengths) if x[1] > maxMessageLength)
+					messages.append(message[:cutoffIndex])
+					message = message[cutoffIndex:]
+					messageB = message.encode('utf-8')
+				for msg in reversed(messages):
+					self.messageQueue.putleft_nowait((channel, msg))
+			else:
+				self.logger.info(f'Sending {message!r} to {channel!r}')
+				self.unconfirmedMessages.append((channel, message))
+				self.send(b'PRIVMSG ' + channel.encode('utf-8') + b' :' + message.encode('utf-8'))
+				await asyncio.sleep(1) # Rate limit
 
 	async def confirm_messages(self):
 		while self.connected:
@@ -438,7 +521,7 @@ class IRCClientProtocol(asyncio.Protocol):
 				self.logger.error('IRC connection registered but not authenticated, terminating connection')
 				self.transport.close()
 				return
-			self.send(b'JOIN ' + ','.join(self.channels).encode('utf-8')) #TODO: Split if too long
+			self._send_join_part(b'JOIN', self.channels)
 			asyncio.create_task(self.send_messages())
 			asyncio.create_task(self.confirm_messages())