diff --git a/atproto/syntax/atidentifier.go b/atproto/syntax/atidentifier.go
new file mode 100644
index 00000000..0fb7da1c
--- /dev/null
+++ b/atproto/syntax/atidentifier.go
@@ -0,0 +1,61 @@
+package syntax
+
+import (
+	"fmt"
+	"strings"
+)
+
+type AtIdentifier struct {
+	Inner interface{}
+}
+
+func ParseAtIdentifier(raw string) (*AtIdentifier, error) {
+	if strings.HasPrefix(raw, "did:") {
+		did, err := ParseDID(raw)
+		if err != nil {
+			return nil, err
+		}
+		return &AtIdentifier{Inner: did}, nil
+	}
+	handle, err := ParseHandle(raw)
+	if err != nil {
+		return nil, err
+	}
+	return &AtIdentifier{Inner: handle}, nil
+}
+
+func (n AtIdentifier) AsHandle() (Handle, error) {
+	handle, ok := n.Inner.(Handle)
+	if ok {
+		return handle, nil
+	}
+	return "", fmt.Errorf("AT Identifier is not a Handle")
+}
+
+func (n AtIdentifier) AsDID() (DID, error) {
+	did, ok := n.Inner.(DID)
+	if ok {
+		return did, nil
+	}
+	return "", fmt.Errorf("AT Identifier is not a DID")
+}
+
+func (n AtIdentifier) Normalize() AtIdentifier {
+	handle, ok := n.Inner.(Handle)
+	if ok {
+		return AtIdentifier{Inner: handle.Normalize()}
+	}
+	return n
+}
+
+func (n AtIdentifier) String() string {
+	did, ok := n.Inner.(DID)
+	if ok {
+		return did.String()
+	}
+	handle, ok := n.Inner.(Handle)
+	if ok {
+		return handle.String()
+	}
+	return ""
+}
diff --git a/atproto/syntax/atidentifier_test.go b/atproto/syntax/atidentifier_test.go
new file mode 100644
index 00000000..7bf1ff4e
--- /dev/null
+++ b/atproto/syntax/atidentifier_test.go
@@ -0,0 +1,50 @@
+package syntax
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestInteropAtIdentifiersValid(t *testing.T) {
+	assert := assert.New(t)
+	file, err := os.Open("testdata/atidentifier_syntax_valid.txt")
+	assert.NoError(err)
+	defer file.Close()
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+		_, err := ParseAtIdentifier(line)
+		if err != nil {
+			fmt.Println("FAILED, GOOD: " + line)
+		}
+		assert.NoError(err)
+	}
+	assert.NoError(scanner.Err())
+}
+
+func TestInteropAtIdentifiersInvalid(t *testing.T) {
+	assert := assert.New(t)
+	file, err := os.Open("testdata/atidentifier_syntax_invalid.txt")
+	assert.NoError(err)
+	defer file.Close()
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		line := scanner.Text()
+		if len(line) == 0 || line[0] == '#' {
+			continue
+		}
+		_, err := ParseAtIdentifier(line)
+		if err == nil {
+			fmt.Println("FAILED, BAD: " + line)
+		}
+		assert.Error(err)
+	}
+	assert.NoError(scanner.Err())
+}
diff --git a/atproto/syntax/testdata/atidentifier_syntax_invalid.txt b/atproto/syntax/testdata/atidentifier_syntax_invalid.txt
new file mode 100644
index 00000000..f0f84309
--- /dev/null
+++ b/atproto/syntax/testdata/atidentifier_syntax_invalid.txt
@@ -0,0 +1,28 @@
+
+# invalid handles
+did:thing.test
+did:thing
+john-.test
+john.0
+john.-
+xn--bcher-.tld
+john..test
+jo_hn.test
+
+# invalid DIDs
+did
+didmethodval
+method:did:val
+did:method:
+didmethod:val
+did:methodval)
+:did:method:val
+did:method:val:
+did:method:val%
+DID:method:val
+
+# other invalid stuff
+email@example.com
+@handle@example.com
+@handle
+blah
diff --git a/atproto/syntax/testdata/atidentifier_syntax_valid.txt b/atproto/syntax/testdata/atidentifier_syntax_valid.txt
new file mode 100644
index 00000000..cc4a42b0
--- /dev/null
+++ b/atproto/syntax/testdata/atidentifier_syntax_valid.txt
@@ -0,0 +1,15 @@
+
+# allows valid handles
+XX.LCS.MIT.EDU
+john.test
+jan.test
+a234567890123456789.test
+john2.test
+john-john.test
+
+# allows valid DIDs
+did:method:val
+did:method:VAL
+did:method:val123
+did:method:123
+did:method:val-two