diff --git a/Changes.md b/Changes.md
index 3c9e0e3..bf31312 100644
--- a/Changes.md
+++ b/Changes.md
@@ -1,3 +1,9 @@
+WIP  TBD
+
+ * Added maps.MergeInPlace.
+ * Added slices.MapSlice and slices.MapMap.
+ * maps.Merge now allocates room equal to the size of all input maps in the output map.
+
 v0.3.0  2023-07-13
 
  * Added slices.FirstIndex and slices.GrepIndex.
diff --git a/README.md b/README.md
index e957237..01a55d8 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,8 @@ off-by-one errors, so the following ops are provided:
  * `Unshift(slice, value)`
  * `Concat(slice...)`
  * `Map(slice, mapper)`
+ * `MapSlice(slice, mapper)`
+ * `MapMap(slice, mapper)`
  * `Reduce(slice, reducer)`
  * `Reductions(slice, reducer)`
  * `ReduceAcc(slice, start, reducer)`
@@ -87,6 +89,7 @@ There are a lot of common map operations that are missing from the built-in
 standard library:
 
  * `Merge(...maps)`
+ * `MergeInPlace(base, ...maps)`
  * `Diff(a, b)`
 
 ## Generic Sets
diff --git a/maps/merge.go b/maps/merge.go
index 1e1abb4..28f8308 100644
--- a/maps/merge.go
+++ b/maps/merge.go
@@ -1,17 +1,31 @@
 package maps
 
-import "github.com/zostay/go-std/generic"
+import (
+	"github.com/zostay/go-std/generic"
+)
 
-// Merge maps into a single map. Keys in maps later in the list will overwrite
-// keys found in earlier maps.
+// Merge maps many maps into a single map. Keys in maps later in the list will
+// overwrite keys found in earlier maps. The returned map will be a newly
+// allocated map.
 func Merge[K comparable, V any](maps ...map[K]V) map[K]V {
-	out := map[K]V{}
+	size := 0
 	for _, m := range maps {
+		size += len(m)
+	}
+	return MergeInPlace(make(map[K]V, size), maps...)
+}
+
+// MergeInPlace maps many maps into a single map. The base map will be updated
+// to include all the pairs in the merge maps. Each map will be itereated in
+// order with values in later maps overwriting those in early maps. When
+// complete, the base map is returned.
+func MergeInPlace[K comparable, V any](base map[K]V, merge ...map[K]V) map[K]V {
+	for _, m := range merge {
 		for k, v := range m {
-			out[k] = v
+			base[k] = v
 		}
 	}
-	return out
+	return base
 }
 
 // Diff returns three maps from two. The first map returned is a map of
diff --git a/maps/merge_test.go b/maps/merge_test.go
index 573453d..7a64496 100644
--- a/maps/merge_test.go
+++ b/maps/merge_test.go
@@ -28,6 +28,23 @@ func TestMerge(t *testing.T) {
 	}, maps.Merge(a, b, c))
 }
 
+func TestMergeInPlace(t *testing.T) {
+	a := map[string]int{"a": 1, "b": 2}
+	b := map[string]int{"c": 3, "d": 4}
+	c := map[string]int{"a": 5, "b": 6, "c": 7}
+
+	assert.Equal(t, map[string]int{}, maps.MergeInPlace(map[string]int{}))
+	assert.Equal(t, map[string]int{
+		"a": 1, "b": 2,
+	}, maps.MergeInPlace(a))
+	assert.Equal(t, map[string]int{
+		"a": 1, "b": 2, "c": 3, "d": 4,
+	}, maps.MergeInPlace(a, b))
+	assert.Equal(t, map[string]int{
+		"a": 5, "b": 6, "c": 7, "d": 4,
+	}, maps.MergeInPlace(a, c))
+}
+
 func TestDiff(t *testing.T) {
 	a := map[string]int{"one": 1, "two": 2, "three": 3, "four": 4}
 	b := map[string]int{"one": 5, "three": 6, "five": 7, "seven": 8}
diff --git a/slices/map.go b/slices/map.go
index 3e6c46a..8dd7071 100644
--- a/slices/map.go
+++ b/slices/map.go
@@ -1,6 +1,9 @@
 package slices
 
-import "github.com/zostay/go-std/generic"
+import (
+	"github.com/zostay/go-std/generic"
+	"github.com/zostay/go-std/maps"
+)
 
 // Map transforms the input into the output value using the mapper function.
 func Map[In, Out any](in []In, mapper func(in In) Out) []Out {
@@ -11,6 +14,30 @@ func Map[In, Out any](in []In, mapper func(in In) Out) []Out {
 	return out
 }
 
+// MapSlice transforms the input into an output slice using the mapper function.
+// A single input value may map to multiple or zero output values.
+func MapSlice[In, Out any](in []In, mapper func(in In) []Out) []Out {
+	out := make([]Out, 0, len(in))
+	for _, t := range in {
+		out = append(out, mapper(t)...)
+	}
+	return out
+}
+
+// MapMap transforms the input into an output map using the mapper function.
+// This will allocate a master map. The values returned by each call to mapper
+// for each item in the input argument named in will be added to that map. If
+// keys are added later that have already been added to the map being built, the
+// new values will overwrite the old.
+func MapMap[In any, K comparable, V any](in []In, mapper func(in In) map[K]V) map[K]V {
+	out := make(map[K]V, len(in))
+	for _, t := range in {
+		outMap := mapper(t)
+		out = maps.Merge(out, outMap)
+	}
+	return out
+}
+
 // Reduce collapses the input slice into a single value using the reducer
 // function.
 func Reduce[In, Out any](in []In, reducer func(u Out, t In) Out) Out {
diff --git a/slices/map_test.go b/slices/map_test.go
index 7899bc4..e6b86ed 100644
--- a/slices/map_test.go
+++ b/slices/map_test.go
@@ -45,6 +45,35 @@ func TestMap(t *testing.T) {
 	assert.Equal(t, []string{}, ss)
 }
 
+func TestMapSlice(t *testing.T) {
+	halveTheOdds := func(i int) []float64 {
+		if i%2 == 1 {
+			return []float64{float64(i) / 2.0}
+		} else {
+			return []float64{}
+		}
+	}
+
+	in := []int{1, 2, 3}
+	ss := slices.MapSlice(in, halveTheOdds)
+	assert.Equal(t, []float64{0.5, 1.5}, ss)
+	ss = slices.MapSlice([]int{}, halveTheOdds)
+	assert.Equal(t, []float64{}, ss)
+}
+
+func TestMapMap(t *testing.T) {
+	stringToInt := func(in int) map[string]int {
+		return map[string]int{
+			strconv.Itoa(in): in,
+		}
+	}
+	in := []int{1, 2, 3}
+	ss := slices.MapMap(in, stringToInt)
+	assert.Equal(t, map[string]int{"1": 1, "2": 2, "3": 3}, ss)
+	ss = slices.MapMap([]int{}, stringToInt)
+	assert.Equal(t, map[string]int{}, ss)
+}
+
 func TestReduce(t *testing.T) {
 	in := []int{1, 2, 3}
 	s := slices.Reduce(in, subtract)