diff --git a/eventV1.go b/eventV1.go
index 1404b058..39081b87 100644
--- a/eventV1.go
+++ b/eventV1.go
@@ -144,6 +144,10 @@ func (e *eventV1) AuthEventIDs() []string {
 	return result
 }
 
+func (e *eventV1) PrevStateEventIDs() []string {
+	panic("not implemented in this room version")
+}
+
 func (e *eventV1) OriginServerTS() spec.Timestamp {
 	return e.eventFields.OriginServerTS
 }
diff --git a/eventV2.go b/eventV2.go
index 55ce556a..63c7624d 100644
--- a/eventV2.go
+++ b/eventV2.go
@@ -15,8 +15,9 @@ import (
 
 type eventV2 struct {
 	eventV1
-	PrevEvents []string `json:"prev_events"`
-	AuthEvents []string `json:"auth_events"`
+	PrevEvents      []string `json:"prev_events"`
+	AuthEvents      []string `json:"auth_events"`
+	PrevStateEvents []string `json:"prev_state_events"`
 }
 
 func (e *eventV2) PrevEventIDs() []string {
@@ -27,6 +28,10 @@ func (e *eventV2) AuthEventIDs() []string {
 	return e.AuthEvents
 }
 
+func (e *eventV2) PrevStateEventIDs() []string {
+	return e.PrevStateEvents
+}
+
 // MarshalJSON implements json.Marshaller
 func (e *eventV2) MarshalJSON() ([]byte, error) {
 	if e.eventJSON == nil {
@@ -185,6 +190,8 @@ func newEventFromUntrustedJSONV2(eventJSON []byte, roomVersion IRoomVersion) (PD
 	return res, err
 }
 
+// This has to exist outside the RoomVersion interface due to init cycles caused
+// when you try to MustGetRoomVersion inside CheckFields. Ideally we'd refactor that...
 var lenientByteLimitRoomVersions = map[RoomVersion]struct{}{
 	RoomVersionV1:        {},
 	RoomVersionV2:        {},
@@ -204,10 +211,24 @@ var lenientByteLimitRoomVersions = map[RoomVersion]struct{}{
 	"org.matrix.msc3667": {},
 }
 
+var stateDAGRoomVersions = map[RoomVersion]struct{}{
+	RoomVersionStateDAGs: {},
+}
+
 func CheckFields(input PDU) error { // nolint: gocyclo
-	if input.AuthEventIDs() == nil || input.PrevEventIDs() == nil {
-		return errors.New("gomatrixserverlib: auth events and prev events must not be nil")
+	// don't check auth event IDs in auth DAG rooms as it doesn't exist.
+	_, stateDAGs := stateDAGRoomVersions[input.Version()]
+	if !stateDAGs && input.AuthEventIDs() == nil {
+		return errors.New("gomatrixserverlib: auth events must not be nil")
 	}
+	if input.PrevEventIDs() == nil {
+		return errors.New("gomatrixserverlib: prev events must not be nil")
+	}
+	if stateDAGs && input.PrevStateEventIDs() == nil {
+		// create event should be []
+		return errors.New("gomatrixserverlib: prev_state_events must not be nil")
+	}
+
 	if l := len(input.JSON()); l > maxEventLength {
 		return EventValidationError{
 			Code:    EventValidationTooLarge,
diff --git a/eventV2_test.go b/eventV2_test.go
index b9d5a6a8..bd8ce0c4 100644
--- a/eventV2_test.go
+++ b/eventV2_test.go
@@ -146,7 +146,7 @@ func TestCheckFields(t *testing.T) {
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			for roomVersion := range roomVersionMeta {
-				if roomVersion == RoomVersionPseudoIDs {
+				if roomVersion == RoomVersionPseudoIDs || roomVersion == RoomVersionStateDAGs {
 					continue
 				}
 				t.Run(tt.name+"-"+string(roomVersion), func(t *testing.T) {
diff --git a/event_builder.go b/event_builder.go
index 28438148..5262f2ef 100644
--- a/event_builder.go
+++ b/event_builder.go
@@ -29,6 +29,8 @@ type EventBuilder struct {
 	// The events needed to authenticate this event. This can be
 	// either []eventReference for room v1/v2, and []string for room v3 onwards.
 	AuthEvents interface{} `json:"auth_events"`
+	// Previous state events, for MSC4242 state dag rooms only. Pointer to allow [] to encode for the create event.
+	PrevStateEvents *[]string `json:"prev_state_events,omitempty"`
 	// The event ID of the event being redacted if this event is a "m.room.redaction".
 	Redacts string `json:"redacts,omitempty"`
 	// The depth of the event, This should be one greater than the maximum depth of the previous events.
@@ -145,6 +147,10 @@ func (eb *EventBuilder) Build(
 	if eb.version == nil {
 		return nil, fmt.Errorf("EventBuilder.Build: unknown version, did you create this via NewEventBuilder?")
 	}
+	isStateDAGs := eb.version.StateDAGs()
+	if !isStateDAGs && eb.PrevStateEvents != nil {
+		return nil, fmt.Errorf("prev_state_events can only be set on RoomVersionStateDAGs")
+	}
 
 	eventFormat := eb.version.EventFormat()
 	eventIDFormat := eb.version.EventIDFormat()
@@ -213,6 +219,12 @@ func (eb *EventBuilder) Build(
 		}
 	}
 
+	if isStateDAGs {
+		if eventJSON, err = sjson.DeleteBytes(eventJSON, "auth_events"); err != nil {
+			return
+		}
+	}
+
 	if eventJSON, err = addContentHashesToEvent(eventJSON); err != nil {
 		return
 	}
diff --git a/eventversion.go b/eventversion.go
index 3dfbdf7c..9f38585d 100644
--- a/eventversion.go
+++ b/eventversion.go
@@ -38,6 +38,7 @@ type IRoomVersion interface {
 	CheckCreateEvent(event PDU, sender spec.UserID, knownRoomVersion KnownRoomVersionFunc) error
 	DomainlessRoomIDs() bool
 	PrivilegedCreators() bool
+	StateDAGs() bool
 }
 
 type KnownRoomVersionFunc func(RoomVersion) bool
@@ -69,6 +70,7 @@ const (
 	RoomVersionV12       RoomVersion = "12"
 	RoomVersionPseudoIDs RoomVersion = "org.matrix.msc4014"
 	RoomVersionHydra     RoomVersion = "org.matrix.hydra.11"
+	RoomVersionStateDAGs RoomVersion = "org.matrix.msc4242.12"
 )
 
 // Event format constants.
@@ -417,6 +419,31 @@ var roomVersionMeta = map[RoomVersion]IRoomVersion{
 		domainlessRoomID:                       true,
 		privilegedCreators:                     true,
 	},
+	// Based on v12
+	RoomVersionStateDAGs: RoomVersionImpl{
+		ver:                            RoomVersionStateDAGs,
+		stable:                         true,
+		stateResAlgorithm:              StateResV2_1,
+		eventFormat:                    EventFormatV2,
+		eventIDFormat:                  EventIDFormatV3,
+		redactionAlgorithm:             redactEventJSONVStateDAGs,
+		signatureValidityCheckFunc:     StrictValiditySignatureCheck,
+		canonicalJSONCheck:             verifyEnforcedCanonicalJSON,
+		checkPowerLevelEvent:           checkPowerLevelEventV3,
+		restrictedJoinServernameFunc:   extractAuthorisedViaServerName,
+		checkRestrictedJoin:            checkRestrictedJoin,
+		parsePowerLevelsFunc:           parseIntegerPowerLevels,
+		checkKnockingAllowedFunc:       checkKnocking,
+		checkRestrictedJoinAllowedFunc: allowRestrictedJoins,
+		checkCreateEvent:               checkCreateEventV3,
+		// v3 versions relax the room ID check as the room ID has no domain now.
+		newEventFromUntrustedJSONFunc:          newEventFromUntrustedJSONV3,
+		newEventFromTrustedJSONFunc:            newEventFromTrustedJSONV3,
+		newEventFromTrustedJSONWithEventIDFunc: newEventFromTrustedJSONWithEventIDV3,
+		domainlessRoomID:                       true,
+		privilegedCreators:                     true,
+		stateDAGs:                              true,
+	},
 }
 
 // RoomVersions returns information about room versions currently
@@ -501,7 +528,9 @@ type RoomVersionImpl struct {
 	// whether auth_events should include the create event
 	domainlessRoomID bool
 	// creators have infinite PL
-	privilegedCreators             bool
+	privilegedCreators bool
+	// Events form two graphs, a state DAG and an event DAG.
+	stateDAGs                      bool
 	checkRestrictedJoin            func(ctx context.Context, localServerName spec.ServerName, roomQuerier RestrictedRoomJoinQuerier, roomID spec.RoomID, senderID spec.SenderID, privilegedCreators bool) (string, error)
 	restrictedJoinServernameFunc   func(content []byte) (spec.ServerName, error)
 	checkRestrictedJoinAllowedFunc func() error
@@ -525,6 +554,10 @@ func (v RoomVersionImpl) DomainlessRoomIDs() bool {
 	return v.domainlessRoomID
 }
 
+func (v RoomVersionImpl) StateDAGs() bool {
+	return v.stateDAGs
+}
+
 func (v RoomVersionImpl) PrivilegedCreators() bool {
 	return v.privilegedCreators
 }
@@ -630,6 +663,7 @@ func (v RoomVersionImpl) NewEventBuilderFromProtoEvent(pe *ProtoEvent) *EventBui
 	eb.StateKey = pe.StateKey
 	eb.Type = pe.Type
 	eb.Unsigned = pe.Unsigned
+	eb.PrevStateEvents = pe.PrevStateEvents
 	return eb
 }
 
diff --git a/fclient/federationtypes.go b/fclient/federationtypes.go
index c46dd0c4..0dc6a9e7 100644
--- a/fclient/federationtypes.go
+++ b/fclient/federationtypes.go
@@ -90,6 +90,8 @@ type MissingEvents struct {
 	EarliestEvents []string `json:"earliest_events"`
 	// The event IDs to retrieve the previous events for.
 	LatestEvents []string `json:"latest_events"`
+	// If true, walks the state DAG. Only for MSC4242 State DAG rooms.
+	StateDAG bool `json:"org.matrix.msc4242.state_dag"`
 }
 
 // A RespMissingEvents is the content of a response to GET /_matrix/federation/v1/get_missing_events/{roomID}
@@ -269,6 +271,8 @@ type RespSendJoin struct {
 	StateEvents gomatrixserverlib.EventJSONs `json:"state"`
 	// A list of events needed to authenticate the state events.
 	AuthEvents gomatrixserverlib.EventJSONs `json:"auth_chain"`
+	// MSC4242: The entire state DAG for the room
+	StateDAG gomatrixserverlib.EventJSONs `json:"state_dag"`
 	// The server that originated the event.
 	Origin spec.ServerName `json:"origin"`
 	// The returned join event from the remote server. Used for restricted joins,
@@ -318,6 +322,9 @@ func (r RespSendJoin) MarshalJSON() ([]byte, error) {
 	if len(fields.StateEvents) == 0 {
 		fields.StateEvents = gomatrixserverlib.EventJSONs{}
 	}
+	if len(r.StateDAG) > 0 {
+		fields.StateDAG = r.StateDAG
+	}
 
 	if !r.MembersOmitted {
 		return json.Marshal(fields)
@@ -350,6 +357,7 @@ type RespMakeKnock struct {
 type respSendJoinFields struct {
 	StateEvents gomatrixserverlib.EventJSONs `json:"state"`
 	AuthEvents  gomatrixserverlib.EventJSONs `json:"auth_chain"`
+	StateDAG    gomatrixserverlib.EventJSONs `json:"state_dag,omitempty"`
 	Origin      spec.ServerName              `json:"origin"`
 	Event       spec.RawJSON                 `json:"event,omitempty"`
 }
diff --git a/join.go b/join.go
index a6483e81..61b799c7 100644
--- a/join.go
+++ b/join.go
@@ -43,7 +43,9 @@ type ProtoEvent struct {
 	PrevEvents interface{} `json:"prev_events"`
 	// The events needed to authenticate this event. This can be
 	// either []eventReference for room v1/v2, and []string for room v3 onwards.
-	AuthEvents interface{} `json:"auth_events"`
+	AuthEvents interface{} `json:"auth_events,omitempty"`
+	// Previous state events, for MSC4242 state dag rooms. Pointer to allow [] to encode for the create event.
+	PrevStateEvents *[]string `json:"prev_state_events,omitempty"`
 	// The event ID of the event being redacted if this event is a "m.room.redaction".
 	Redacts string `json:"redacts,omitempty"`
 	// The depth of the event, This should be one greater than the maximum depth of the previous events.
diff --git a/pdu.go b/pdu.go
index 347323db..bd9debec 100644
--- a/pdu.go
+++ b/pdu.go
@@ -53,6 +53,7 @@ type PDU interface {
 	JSON() []byte                    // TODO: remove
 	AuthEventIDs() []string          // TODO: remove
 	ToHeaderedJSON() ([]byte, error) // TODO: remove
+	PrevStateEventIDs() []string
 	// IsSticky returns true if the event is *currently* considered "sticky" given the received time.
 	// Sticky events are annotated as sticky and carry strong delivery guarantees to clients (and
 	// therefore servers). `received` should be specified as the time the event was received by the
diff --git a/redactevent.go b/redactevent.go
index c69ccb01..163da411 100644
--- a/redactevent.go
+++ b/redactevent.go
@@ -141,7 +141,7 @@ func redactEventJSONV1(eventJSON []byte) ([]byte, error) {
 }
 
 type unredactableEvent interface {
-	*unredactableEventFieldsV1 | *unredactableEventFieldsV2
+	*unredactableEventFieldsV1 | *unredactableEventFieldsV2 | *unredactableEventFieldsVStateDAGs
 	GetType() string
 	GetContent() map[string]interface{}
 	SetContent(map[string]interface{})
@@ -172,3 +172,34 @@ func redactEventJSON[T unredactableEvent](eventJSON []byte, unredactableEvent T,
 	// Return the redacted event encoded as JSON.
 	return json.Marshal(&unredactableEvent)
 }
+
+type unredactableEventFieldsVStateDAGs struct {
+	EventID         spec.RawJSON           `json:"event_id,omitempty"`
+	Type            string                 `json:"type"`
+	RoomID          spec.RawJSON           `json:"room_id,omitempty"`
+	Sender          spec.RawJSON           `json:"sender,omitempty"`
+	StateKey        spec.RawJSON           `json:"state_key,omitempty"`
+	Content         map[string]interface{} `json:"content"`
+	Hashes          spec.RawJSON           `json:"hashes,omitempty"`
+	Signatures      spec.RawJSON           `json:"signatures,omitempty"`
+	Depth           spec.RawJSON           `json:"depth,omitempty"`
+	PrevEvents      spec.RawJSON           `json:"prev_events,omitempty"`
+	PrevStateEvents spec.RawJSON           `json:"prev_state_events,omitempty"`
+	OriginServerTS  spec.RawJSON           `json:"origin_server_ts,omitempty"`
+}
+
+func (u *unredactableEventFieldsVStateDAGs) GetType() string {
+	return u.Type
+}
+
+func (u *unredactableEventFieldsVStateDAGs) GetContent() map[string]interface{} {
+	return u.Content
+}
+
+func (u *unredactableEventFieldsVStateDAGs) SetContent(content map[string]interface{}) {
+	u.Content = content
+}
+
+func redactEventJSONVStateDAGs(eventJSON []byte) ([]byte, error) {
+	return redactEventJSON(eventJSON, &unredactableEventFieldsVStateDAGs{}, unredactableContentFieldsV5)
+}