diff --git a/error.go b/error.go
new file mode 100644
index 0000000..2c4e366
--- /dev/null
+++ b/error.go
@@ -0,0 +1,46 @@
+// Copyright © 2014 Steve Francia <spf@spf13.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package cast
+
+import "fmt"
+
+// Error is a marker interface implemented by all domain errors returned by the library.
+type Error interface {
+	castError()
+}
+
+// InvalidCastError is returned when a value cannot be cast from its type to the target type.
+type InvalidCastError struct {
+	value any
+	typ   string
+}
+
+func (e InvalidCastError) Error() string {
+	// TODO: consider adding the value back
+	// maybe with a length limit?
+	return fmt.Sprintf("unable to cast from type %T to %s", e.value, e.typ)
+}
+
+// Implements the [Error] marker interface.
+func (InvalidCastError) castError() {}
+
+// RangeError is returned when a value cannot be cast from its type to the target type.
+type RangeError struct {
+	value any
+	typ   string
+
+	min any
+	max any
+}
+
+func (e RangeError) Error() string {
+	// TODO: consider adding the value back
+	// maybe with a length limit?
+	return fmt.Sprintf("value of type %T is out of range for %s [%v, %v]", e.value, e.typ, e.min, e.max)
+}
+
+// Implements the [Error] marker interface.
+func (RangeError) castError() {}