diff --git a/irc/channel.go b/irc/channel.go
index 498d6d45..a38ebca3 100644
--- a/irc/channel.go
+++ b/irc/channel.go
@@ -76,7 +76,7 @@ func NewChannel(s *Server, name string, registered bool) *Channel {
 	config := s.Config()
 
 	channel.writerSemaphore.Initialize(1)
-	channel.history.Initialize(config.History.ChannelLength)
+	channel.history.Initialize(config.History.ChannelLength, config.History.AutoresizeWindow)
 
 	if !registered {
 		for _, mode := range config.Channels.defaultModes {
diff --git a/irc/client.go b/irc/client.go
index c69ed9bb..00200198 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -232,7 +232,7 @@ func (server *Server) RunClient(conn clientConn) {
 		nickCasefolded: "*",
 		nickMaskString: "*", // * is used until actual nick is given
 	}
-	client.history.Initialize(config.History.ClientLength)
+	client.history.Initialize(config.History.ClientLength, config.History.AutoresizeWindow)
 	client.brbTimer.Initialize(client)
 	session := &Session{
 		client:     client,
diff --git a/irc/config.go b/irc/config.go
index 9423e13e..70062c58 100644
--- a/irc/config.go
+++ b/irc/config.go
@@ -353,10 +353,11 @@ type Config struct {
 
 	History struct {
 		Enabled          bool
-		ChannelLength    int `yaml:"channel-length"`
-		ClientLength     int `yaml:"client-length"`
-		AutoreplayOnJoin int `yaml:"autoreplay-on-join"`
-		ChathistoryMax   int `yaml:"chathistory-maxmessages"`
+		ChannelLength    int           `yaml:"channel-length"`
+		ClientLength     int           `yaml:"client-length"`
+		AutoresizeWindow time.Duration `yaml:"autoresize-window"`
+		AutoreplayOnJoin int           `yaml:"autoreplay-on-join"`
+		ChathistoryMax   int           `yaml:"chathistory-maxmessages"`
 	}
 
 	Filename string
diff --git a/irc/history/history.go b/irc/history/history.go
index 790eae12..5f604c23 100644
--- a/irc/history/history.go
+++ b/irc/history/history.go
@@ -25,6 +25,10 @@ const (
 	Nick
 )
 
+const (
+	initialAutoSize = 32
+)
+
 // a Tagmsg that consists entirely of transient tags is not stored
 var transientTags = map[string]bool{
 	"+draft/typing": true,
@@ -77,25 +81,39 @@ type Buffer struct {
 	sync.RWMutex
 
 	// ring buffer, see irc/whowas.go for conventions
-	buffer []Item
-	start  int
-	end    int
+	buffer      []Item
+	start       int
+	end         int
+	maximumSize int
+	window      time.Duration
 
 	lastDiscarded time.Time
 
 	enabled uint32
+
+	nowFunc func() time.Time
 }
 
-func NewHistoryBuffer(size int) (result *Buffer) {
+func NewHistoryBuffer(size int, window time.Duration) (result *Buffer) {
 	result = new(Buffer)
-	result.Initialize(size)
+	result.Initialize(size, window)
 	return
 }
 
-func (hist *Buffer) Initialize(size int) {
-	hist.buffer = make([]Item, size)
+func (hist *Buffer) Initialize(size int, window time.Duration) {
+	initialSize := size
+	if window != 0 {
+		initialSize = initialAutoSize
+		if size < initialSize {
+			initialSize = size // min(initialAutoSize, size)
+		}
+	}
+	hist.buffer = make([]Item, initialSize)
 	hist.start = -1
 	hist.end = -1
+	hist.window = window
+	hist.maximumSize = size
+	hist.nowFunc = time.Now
 
 	hist.setEnabled(size)
 }
@@ -132,6 +150,8 @@ func (list *Buffer) Add(item Item) {
 	list.Lock()
 	defer list.Unlock()
 
+	list.maybeExpand()
+
 	var pos int
 	if list.start == -1 { // empty
 		pos = 0
@@ -269,12 +289,68 @@ func (list *Buffer) next(index int) int {
 	}
 }
 
+// return n such that v <= n and n == 2**i for some i
+func roundUpToPowerOfTwo(v int) int {
+	// http://graphics.stanford.edu/~seander/bithacks.html
+	v -= 1
+	v |= v >> 1
+	v |= v >> 2
+	v |= v >> 4
+	v |= v >> 8
+	v |= v >> 16
+	return v + 1
+}
+
+func (list *Buffer) maybeExpand() {
+	if list.window == 0 {
+		return // autoresize is disabled
+	}
+
+	length := list.length()
+	if length < len(list.buffer) {
+		return // we have spare capacity already
+	}
+
+	if len(list.buffer) == list.maximumSize {
+		return // cannot expand any further
+	}
+
+	wouldDiscard := list.buffer[list.start].Message.Time
+	if list.window < list.nowFunc().Sub(wouldDiscard) {
+		return // oldest element is old enough to overwrite
+	}
+
+	newSize := roundUpToPowerOfTwo(length + 1)
+	if list.maximumSize < newSize {
+		newSize = list.maximumSize
+	}
+	list.resize(newSize)
+}
+
 // Resize shrinks or expands the buffer
-func (list *Buffer) Resize(size int) {
-	newbuffer := make([]Item, size)
+func (list *Buffer) Resize(maximumSize int, window time.Duration) {
 	list.Lock()
 	defer list.Unlock()
 
+	if list.maximumSize == maximumSize && list.window == window {
+		return // no-op
+	}
+
+	list.maximumSize = maximumSize
+	list.window = window
+
+	// if we're not autoresizing, we need to resize now;
+	// if we are autoresizing, we may need to shrink the buffer down to maximumSize,
+	// but we don't need to grow it now (we can just grow it on the next Add)
+	// TODO make it possible to shrink the buffer so that it only contains `window`
+	if window == 0 || maximumSize < len(list.buffer) {
+		list.resize(maximumSize)
+	}
+}
+
+func (list *Buffer) resize(size int) {
+	newbuffer := make([]Item, size)
+
 	list.setEnabled(size)
 
 	if list.start == -1 {
diff --git a/irc/history/history_test.go b/irc/history/history_test.go
index 451f331b..10364437 100644
--- a/irc/history/history_test.go
+++ b/irc/history/history_test.go
@@ -5,6 +5,7 @@ package history
 
 import (
 	"reflect"
+	"strconv"
 	"testing"
 	"time"
 )
@@ -16,7 +17,7 @@ const (
 func TestEmptyBuffer(t *testing.T) {
 	pastTime := easyParse(timeFormat)
 
-	buf := NewHistoryBuffer(0)
+	buf := NewHistoryBuffer(0, 0)
 	if buf.Enabled() {
 		t.Error("the buffer of size 0 must be considered disabled")
 	}
@@ -33,7 +34,7 @@ func TestEmptyBuffer(t *testing.T) {
 		t.Error("the empty/disabled buffer should report results as incomplete")
 	}
 
-	buf.Resize(1)
+	buf.Resize(1, 0)
 	if !buf.Enabled() {
 		t.Error("the buffer of size 1 must be considered enabled")
 	}
@@ -102,7 +103,7 @@ func assertEqual(supplied, expected interface{}, t *testing.T) {
 func TestBuffer(t *testing.T) {
 	start := easyParse("2006-01-01 00:00:00Z")
 
-	buf := NewHistoryBuffer(3)
+	buf := NewHistoryBuffer(3, 0)
 	buf.Add(easyItem("testnick0", "2006-01-01 15:04:05Z"))
 
 	buf.Add(easyItem("testnick1", "2006-01-02 15:04:05Z"))
@@ -128,12 +129,12 @@ func TestBuffer(t *testing.T) {
 	assertEqual(toNicks(since), []string{"testnick1"}, t)
 
 	// shrink the buffer, cutting off testnick1
-	buf.Resize(2)
+	buf.Resize(2, 0)
 	since, complete = buf.Between(easyParse("2006-01-02 00:00:00Z"), time.Now(), false, 0)
 	assertEqual(complete, false, t)
 	assertEqual(toNicks(since), []string{"testnick2", "testnick3"}, t)
 
-	buf.Resize(5)
+	buf.Resize(5, 0)
 	buf.Add(easyItem("testnick4", "2006-01-05 15:04:05Z"))
 	buf.Add(easyItem("testnick5", "2006-01-06 15:04:05Z"))
 	buf.Add(easyItem("testnick6", "2006-01-07 15:04:05Z"))
@@ -145,3 +146,80 @@ func TestBuffer(t *testing.T) {
 	since, _ = buf.Between(easyParse("2006-01-03 00:00:00Z"), time.Now(), true, 2)
 	assertEqual(toNicks(since), []string{"testnick2", "testnick3"}, t)
 }
+
+func autoItem(id int, t time.Time) (result Item) {
+	result.Message.Time = t
+	result.Nick = strconv.Itoa(id)
+	return
+}
+
+func atoi(s string) int {
+	result, err := strconv.Atoi(s)
+	if err != nil {
+		panic(err)
+	}
+	return result
+}
+
+func TestAutoresize(t *testing.T) {
+	now := easyParse("2006-01-01 00:00:00Z")
+	nowFunc := func() time.Time {
+		return now
+	}
+
+	buf := NewHistoryBuffer(128, time.Hour)
+	buf.nowFunc = nowFunc
+
+	// add items slowly (one every 10 minutes): the buffer should not expand
+	// beyond initialAutoSize
+	id := 0
+	for i := 0; i < 72; i += 1 {
+		buf.Add(autoItem(id, now))
+		if initialAutoSize < buf.length() {
+			t.Errorf("buffer incorrectly resized above %d to %d", initialAutoSize, buf.length())
+		}
+		now = now.Add(time.Minute * 10)
+		id += 1
+	}
+	items := buf.Latest(0)
+	assertEqual(len(items), initialAutoSize, t)
+	assertEqual(atoi(items[0].Nick), 40, t)
+	assertEqual(atoi(items[len(items)-1].Nick), 71, t)
+
+	// dump 100 items in very fast:
+	for i := 0; i < 100; i += 1 {
+		buf.Add(autoItem(id, now))
+		now = now.Add(time.Second)
+		id += 1
+	}
+	// ok, 5 items from the first batch are still in the 1-hour window;
+	// we should overwrite until only those 5 are left, then start expanding
+	// the buffer so that it retains those 5 and the 100 new items
+	items = buf.Latest(0)
+	assertEqual(len(items), 105, t)
+	assertEqual(atoi(items[0].Nick), 67, t)
+	assertEqual(atoi(items[len(items)-1].Nick), 171, t)
+
+	// another 100 items very fast:
+	for i := 0; i < 100; i += 1 {
+		buf.Add(autoItem(id, now))
+		now = now.Add(time.Second)
+		id += 1
+	}
+	// should fill up to the maximum size of 128 and start overwriting
+	items = buf.Latest(0)
+	assertEqual(len(items), 128, t)
+	assertEqual(atoi(items[0].Nick), 144, t)
+	assertEqual(atoi(items[len(items)-1].Nick), 271, t)
+}
+
+func TestRoundUp(t *testing.T) {
+	assertEqual(roundUpToPowerOfTwo(2), 2, t)
+	assertEqual(roundUpToPowerOfTwo(3), 4, t)
+	assertEqual(roundUpToPowerOfTwo(64), 64, t)
+	assertEqual(roundUpToPowerOfTwo(65), 128, t)
+	assertEqual(roundUpToPowerOfTwo(100), 128, t)
+	assertEqual(roundUpToPowerOfTwo(1000), 1024, t)
+	assertEqual(roundUpToPowerOfTwo(1025), 2048, t)
+	assertEqual(roundUpToPowerOfTwo(269435457), 536870912, t)
+}
diff --git a/irc/server.go b/irc/server.go
index 938bdb58..aba91c6e 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -697,16 +697,12 @@ func (server *Server) applyConfig(config *Config, initial bool) (err error) {
 	}
 
 	// resize history buffers as needed
-	if oldConfig != nil {
-		if oldConfig.History.ChannelLength != config.History.ChannelLength {
-			for _, channel := range server.channels.Channels() {
-				channel.history.Resize(config.History.ChannelLength)
-			}
+	if oldConfig != nil && oldConfig.History != config.History {
+		for _, channel := range server.channels.Channels() {
+			channel.history.Resize(config.History.ChannelLength, config.History.AutoresizeWindow)
 		}
-		if oldConfig.History.ClientLength != config.History.ClientLength {
-			for _, client := range server.clients.AllClients() {
-				client.history.Resize(config.History.ClientLength)
-			}
+		for _, client := range server.clients.AllClients() {
+			client.history.Resize(config.History.ClientLength, config.History.AutoresizeWindow)
 		}
 	}
 
diff --git a/oragono.yaml b/oragono.yaml
index d56c1da2..5c3fe2e3 100644
--- a/oragono.yaml
+++ b/oragono.yaml
@@ -593,10 +593,18 @@ history:
     enabled: false
 
     # how many channel-specific events (messages, joins, parts) should be tracked per channel?
-    channel-length: 256
+    channel-length: 1024
 
     # how many direct messages and notices should be tracked per user?
-    client-length: 64
+    client-length: 256
+
+    # how long should we try to preserve messages?
+    # if `autoresize-window` is 0, the in-memory message buffers are preallocated to
+    # their maximum length. if it is nonzero, the buffers are initially small and
+    # are dynamically expanded up to the maximum length. if the buffer is full
+    # and the oldest message is older than `autoresize-window`, then it will overwrite
+    # the oldest message rather than resize; otherwise, it will expand if possible.
+    autoresize-window: 1h
 
     # number of messages to automatically play back on channel join (0 to disable):
     autoreplay-on-join: 0