diff --git a/inlining_test.go b/inlining_test.go
index b3611e3..267fa3a 100644
--- a/inlining_test.go
+++ b/inlining_test.go
@@ -43,8 +43,9 @@ func TestInlining(t *testing.T) {
 	})
 	for _, want := range []string{
 		"(*IPSet).Add",
+		"(*IPSet).Clone",
 		"(*IPSet).Remove",
-		"(*IPSet).RemoveRange",
+		"(*IPSet).removeRangeLocked",
 		"(*uint128).halves",
 		"IP.BitLen",
 		"IP.IPAddr",
@@ -60,6 +61,7 @@ func TestInlining(t *testing.T) {
 		"IP.Prior",
 		"IP.Unmap",
 		"IP.Zone",
+		"IP.lessOrEq",
 		"IP.v4",
 		"IP.v6",
 		"IP.v6u16",
@@ -74,6 +76,7 @@ func TestInlining(t *testing.T) {
 		"IPPrefix.Masked",
 		"IPPrefix.Valid",
 		"IPRange.Prefixes",
+		"IPRange.entirelyBefore",
 		"IPRange.prefixFrom128AndBits",
 		"IPRange.prefixFrom128AndBits-fm",
 		"IPv4",
diff --git a/ipset.go b/ipset.go
index a98caaa..40ef9f4 100644
--- a/ipset.go
+++ b/ipset.go
@@ -4,7 +4,10 @@
 
 package netaddr
 
-import "sort"
+import (
+	"sort"
+	"sync"
+)
 
 // IPSet represents a set of IP addresses.
 //
@@ -15,25 +18,24 @@ import "sort"
 // nothing on an empty set. Ranges may be fully, partially, or not
 // overlapping.
 type IPSet struct {
+	mu sync.Mutex
+
 	// in are the ranges in the set.
 	in []IPRange
 
 	// out are the ranges to be removed from 'in'.
 	out []IPRange
-}
 
-// toInOnly updates s to clear s.out, by merging any s.out into s.in.
-func (s *IPSet) toInOnly() {
-	if len(s.out) > 0 {
-		s.in = s.Ranges()
-		s.out = nil
-	}
+	// normalized indicates that 'in' is in sorted order, and 'out' is
+	// empty.
+	normalized bool
 }
 
 // Clone returns a copy of s that shares no memory with s.
 func (s *IPSet) Clone() *IPSet {
 	return &IPSet{
-		in: s.Ranges(),
+		in:         s.Ranges(),
+		normalized: true,
 	}
 }
 
@@ -45,13 +47,22 @@ func (s *IPSet) AddPrefix(p IPPrefix) { s.AddRange(p.Range()) }
 
 // AddRange adds r to s.
 func (s *IPSet) AddRange(r IPRange) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.addRangeLocked(r)
+}
+
+func (s *IPSet) addRangeLocked(r IPRange) {
 	if !r.Valid() {
 		return
 	}
 	// If there are any removals (s.out), then we need to compact the set
 	// first to get the order right.
-	s.toInOnly()
+	if len(s.out) > 0 {
+		s.rangesLocked()
+	}
 	s.in = append(s.in, r)
+	s.normalized = false
 }
 
 // Remove removes ip from the set s.
@@ -59,7 +70,9 @@ func (s *IPSet) Remove(ip IP) { s.RemoveRange(IPRange{ip, ip}) }
 
 // RemoveFreePrefix removes and returns a Prefix of length bits from the IPSet.
 func (s *IPSet) RemoveFreePrefix(bitLen uint8) (p IPPrefix, ok bool) {
-	prefixes := s.Prefixes()
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	prefixes := s.prefixesLocked()
 	if len(prefixes) == 0 {
 		return IPPrefix{}, false
 	}
@@ -83,7 +96,7 @@ func (s *IPSet) RemoveFreePrefix(bitLen uint8) (p IPPrefix, ok bool) {
 	}
 
 	prefix := IPPrefix{IP: bestFit.IP, Bits: bitLen}
-	s.RemovePrefix(prefix)
+	s.removeRangeLocked(prefix.Range())
 	return prefix, true
 }
 
@@ -92,34 +105,49 @@ func (s *IPSet) RemovePrefix(p IPPrefix) { s.RemoveRange(p.Range()) }
 
 // RemoveRange removes r from s.
 func (s *IPSet) RemoveRange(r IPRange) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.removeRangeLocked(r)
+}
+
+func (s *IPSet) removeRangeLocked(r IPRange) {
 	if r.Valid() {
 		s.out = append(s.out, r)
+		s.normalized = false
 	}
 }
 
 // AddSet adds all ranges in b to s.
 func (s *IPSet) AddSet(b *IPSet) {
-	for _, r := range b.Ranges() {
-		s.AddRange(r)
+	rr := b.Ranges()
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for _, r := range rr {
+		s.addRangeLocked(r)
 	}
 }
 
 // RemoveSet removes all ranges in b from s.
 func (s *IPSet) RemoveSet(b *IPSet) {
-	for _, r := range b.Ranges() {
-		s.RemoveRange(r)
+	rr := b.Ranges()
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for _, r := range rr {
+		s.removeRangeLocked(r)
 	}
 }
 
 // Complement updates s to contain the complement of its current
 // contents.
 func (s *IPSet) Complement() {
-	s.toInOnly()
-	s.out = s.in
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.out = s.rangesLocked()
 	s.in = []IPRange{
 		IPPrefix{IP: IPv4(0, 0, 0, 0), Bits: 0}.Range(),
 		IPPrefix{IP: IPv6Unspecified(), Bits: 0}.Range(),
 	}
+	s.normalized = false
 }
 
 // Intersect updates s to the set intersection of s and b.
@@ -154,6 +182,16 @@ var debugf = discardf
 // Ranges returns the minimum and sorted set of IP
 // ranges that covers s.
 func (s *IPSet) Ranges() []IPRange {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.rangesLocked()
+}
+
+func (s *IPSet) rangesLocked() []IPRange {
+	if s.normalized {
+		return s.in
+	}
+
 	const debug = false
 	if debug {
 		debugf("ranges start in=%v out=%v", s.in, s.out)
@@ -274,7 +312,9 @@ func (s *IPSet) Ranges() []IPRange {
 		}
 	}
 
-	// TODO: possibly update s.in and s.out, if #110 supports that.
+	s.in = ret
+	s.out = nil
+	s.normalized = true
 
 	return ret
 }
@@ -284,8 +324,14 @@ func (s *IPSet) Ranges() []IPRange {
 // returning a new slice of prefixes that covers all of the given 'add'
 // prefixes with all the 'remove' prefixes removed.
 func (s *IPSet) Prefixes() []IPPrefix {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.prefixesLocked()
+}
+
+func (s *IPSet) prefixesLocked() []IPPrefix {
 	var out []IPPrefix
-	for _, r := range s.Ranges() {
+	for _, r := range s.rangesLocked() {
 		out = append(out, r.Prefixes()...)
 	}
 	return out
diff --git a/ipset_test.go b/ipset_test.go
index 092b25f..70e3166 100644
--- a/ipset_test.go
+++ b/ipset_test.go
@@ -481,11 +481,11 @@ func TestIPSetOverlaps(t *testing.T) {
 	for _, test := range tests {
 		got := test.a.Overlaps(test.b)
 		if got != test.want {
-			t.Errorf("(%s).Overlaps(%s) = %v, want %v", test.a, test.b, got, test.want)
+			t.Errorf("(%s).Overlaps(%s) = %v, want %v", test.a.Ranges(), test.b.Ranges(), got, test.want)
 		}
 		got = test.b.Overlaps(test.a)
 		if got != test.want {
-			t.Errorf("(%s).Overlaps(%s) = %v, want %v", test.b, test.a, got, test.want)
+			t.Errorf("(%s).Overlaps(%s) = %v, want %v", test.b.Ranges(), test.a.Ranges(), got, test.want)
 		}
 	}
 }