diff --git a/README.md b/README.md
index 1d4aef3..388e450 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,11 @@ These use strings (lists of characters) for filenames.
 - `is_list/1`
 - `atomic_list_concat/3`
 
+### Atoms
+
+- `downcase_atom/2`
+- `upcase_atom/2`
+
 ### Package [`taujson`](https://godoc.org/github.com/guregu/predicates/taujson)
 
 These predicates are intended to be compatible with Tau Prolog's [`library(js)`](http://tau-prolog.org/documentation#js).
diff --git a/atom.go b/atom.go
new file mode 100644
index 0000000..ab1ca80
--- /dev/null
+++ b/atom.go
@@ -0,0 +1,58 @@
+package predicates
+
+import (
+	"context"
+	"strings"
+
+	"github.com/ichiban/prolog/engine"
+)
+
+// DowncaseAtom (downcase_atom/2) converts atom into its lowercase equivalent.
+//
+//	downcase_atom(+Atom, -LowerCase).
+func DowncaseAtom(atom, lowercase engine.Term, k func(*engine.Env) *engine.Promise, env *engine.Env) *engine.Promise {
+	var a engine.Atom
+	switch atom := env.Resolve(atom).(type) {
+	case engine.Variable:
+		return engine.Error(engine.ErrInstantiation)
+	case engine.Atom:
+		a = atom
+	default:
+		return engine.Error(engine.TypeErrorAtom(atom))
+	}
+
+	switch low := env.Resolve(lowercase).(type) {
+	case engine.Atom, engine.Variable:
+		transformed := engine.Atom(strings.ToLower(string(a)))
+		return engine.Delay(func(context.Context) *engine.Promise {
+			return engine.Unify(low, transformed, k, env)
+		})
+	default:
+		return engine.Error(engine.TypeErrorAtom(low))
+	}
+}
+
+// UpcaseAtom (upcase_atom/2) converts atom into its uppercase equivalent.
+//
+//	upcase_atom(+Atom, -UpperCase).
+func UpcaseAtom(atom, uppercase engine.Term, k func(*engine.Env) *engine.Promise, env *engine.Env) *engine.Promise {
+	var a engine.Atom
+	switch atom := env.Resolve(atom).(type) {
+	case engine.Variable:
+		return engine.Error(engine.ErrInstantiation)
+	case engine.Atom:
+		a = atom
+	default:
+		return engine.Error(engine.TypeErrorAtom(atom))
+	}
+
+	switch low := env.Resolve(uppercase).(type) {
+	case engine.Atom, engine.Variable:
+		transformed := engine.Atom(strings.ToUpper(string(a)))
+		return engine.Delay(func(context.Context) *engine.Promise {
+			return engine.Unify(low, transformed, k, env)
+		})
+	default:
+		return engine.Error(engine.TypeErrorAtom(low))
+	}
+}
diff --git a/atom_test.go b/atom_test.go
new file mode 100644
index 0000000..635486b
--- /dev/null
+++ b/atom_test.go
@@ -0,0 +1,54 @@
+package predicates
+
+import (
+	"testing"
+
+	"github.com/guregu/predicates/internal"
+	"github.com/ichiban/prolog/engine"
+)
+
+func TestUpcaseAtom(t *testing.T) {
+	p := internal.NewTestProlog()
+	p.Register2("upcase_atom", UpcaseAtom)
+
+	t.Run("lower is variable", func(t *testing.T) {
+		t.Run("atom is lowercase", p.Expect([]map[string]engine.Term{
+			{"X": engine.Atom("ABC")},
+		}, `upcase_atom('abc', X).`))
+
+		t.Run("atom is uppercase", p.Expect([]map[string]engine.Term{
+			{"X": engine.Atom("ABC")},
+		}, `upcase_atom('ABC', X).`))
+	})
+
+	t.Run("lower is ground", func(t *testing.T) {
+		t.Run("atom is lowercase", p.Expect(internal.TestOK,
+			`upcase_atom('abc', 'ABC'), OK = true.`))
+
+		t.Run("atom is uppercase", p.Expect(internal.TestOK,
+			`upcase_atom('ABC', 'ABC'), OK = true.`))
+	})
+}
+
+func TestDowncaseAtom(t *testing.T) {
+	p := internal.NewTestProlog()
+	p.Register2("downcase_atom", DowncaseAtom)
+
+	t.Run("lower is variable", func(t *testing.T) {
+		t.Run("atom is lowercase", p.Expect([]map[string]engine.Term{
+			{"X": engine.Atom("abc")},
+		}, `downcase_atom('abc', X).`))
+
+		t.Run("atom is uppercase", p.Expect([]map[string]engine.Term{
+			{"X": engine.Atom("abc")},
+		}, `downcase_atom('ABC', X).`))
+	})
+
+	t.Run("lower is ground", func(t *testing.T) {
+		t.Run("atom is lowercase", p.Expect(internal.TestOK,
+			`downcase_atom('abc', 'abc'), OK = true.`))
+
+		t.Run("atom is uppercase", p.Expect(internal.TestOK,
+			`downcase_atom('ABC', 'abc'), OK = true.`))
+	})
+}