diff --git a/src/api/client/context.rs b/src/api/client/context.rs
index 8c9ba64c..ee3a458c 100644
--- a/src/api/client/context.rs
+++ b/src/api/client/context.rs
@@ -93,7 +93,14 @@ pub(crate) async fn get_context_route(
 		.ignore_err()
 		.then(async |mut pdu| {
 			pdu.1.set_unsigned(Some(sender_user));
-			// TODO: bundled aggregations
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations: {e}");
+			}
 			pdu
 		})
 		.ready_filter_map(|item| event_filter(item, filter))
@@ -109,7 +116,14 @@ pub(crate) async fn get_context_route(
 		.ignore_err()
 		.then(async |mut pdu| {
 			pdu.1.set_unsigned(Some(sender_user));
-			// TODO: bundled aggregations
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations: {e}");
+			}
 			pdu
 		})
 		.ready_filter_map(|item| event_filter(item, filter))
diff --git a/src/api/client/message.rs b/src/api/client/message.rs
index cd6b0a60..6087478c 100644
--- a/src/api/client/message.rs
+++ b/src/api/client/message.rs
@@ -2,7 +2,7 @@ use core::panic;
 
 use axum::extract::State;
 use conduwuit::{
-	Err, Result, at,
+	Err, Result, at, debug_warn,
 	matrix::{
 		Event,
 		pdu::{PduCount, PduEvent},
@@ -134,7 +134,14 @@ pub(crate) async fn get_message_events_route(
 		.take(limit)
 		.then(async |mut pdu| {
 			pdu.1.set_unsigned(Some(sender_user));
-			// TODO: bundled aggregations
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations: {e}");
+			}
 			pdu
 		})
 		.collect()
diff --git a/src/api/client/relations.rs b/src/api/client/relations.rs
index 5e323756..377f0c71 100644
--- a/src/api/client/relations.rs
+++ b/src/api/client/relations.rs
@@ -1,6 +1,6 @@
 use axum::extract::State;
 use conduwuit::{
-	Result, at,
+	Result, at, debug_warn,
 	matrix::pdu::PduCount,
 	utils::{IterStream, ReadyExt, result::FlatOk, stream::WidebandExt},
 };
@@ -149,6 +149,17 @@ async fn paginate_relations_with_filter(
 		.ready_take_while(|(count, _)| Some(*count) != to)
 		.wide_filter_map(|item| visibility_filter(services, sender_user, item))
 		.take(limit)
+		.then(async |mut pdu| {
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations to relation: {e}");
+			}
+			pdu
+		})
 		.collect()
 		.await;
 
diff --git a/src/api/client/room/event.rs b/src/api/client/room/event.rs
index ec673d75..e7b0bb37 100644
--- a/src/api/client/room/event.rs
+++ b/src/api/client/room/event.rs
@@ -1,5 +1,5 @@
 use axum::extract::State;
-use conduwuit::{Err, Event, Result, err};
+use conduwuit::{Err, Event, Result, debug_warn, err};
 use futures::{FutureExt, TryFutureExt, future::try_join};
 use ruma::api::client::room::get_room_event;
 
@@ -38,6 +38,15 @@ pub(crate) async fn get_room_event_route(
 		"Fetched PDU must match requested"
 	);
 
+	if let Err(e) = services
+		.rooms
+		.pdu_metadata
+		.add_bundled_aggregations_to_pdu(body.sender_user(), &mut event)
+		.await
+	{
+		debug_warn!("Failed to add bundled aggregations to event: {e}");
+	}
+
 	event.set_unsigned(body.sender_user.as_deref());
 
 	Ok(get_room_event::v3::Response { event: event.into_room_event() })
diff --git a/src/api/client/room/initial_sync.rs b/src/api/client/room/initial_sync.rs
index 765d2a39..2f965245 100644
--- a/src/api/client/room/initial_sync.rs
+++ b/src/api/client/room/initial_sync.rs
@@ -1,6 +1,6 @@
 use axum::extract::State;
 use conduwuit::{
-	Err, PduEvent, Result, at,
+	Err, PduEvent, Result, at, debug_warn,
 	utils::{BoolExt, stream::TryTools},
 };
 use futures::TryStreamExt;
@@ -35,7 +35,16 @@ pub(crate) async fn room_initial_sync_route(
 		.try_take(limit)
 		.and_then(async |mut pdu| {
 			pdu.1.set_unsigned(body.sender_user.as_deref());
-			// TODO: bundled aggregations
+			if let Some(sender_user) = body.sender_user.as_deref() {
+				if let Err(e) = services
+					.rooms
+					.pdu_metadata
+					.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+					.await
+				{
+					debug_warn!("Failed to add bundled aggregations: {e}");
+				}
+			}
 			Ok(pdu)
 		})
 		.try_collect()
diff --git a/src/api/client/search.rs b/src/api/client/search.rs
index d4dcde57..af5fccec 100644
--- a/src/api/client/search.rs
+++ b/src/api/client/search.rs
@@ -2,7 +2,7 @@ use std::collections::BTreeMap;
 
 use axum::extract::State;
 use conduwuit::{
-	Err, Result, at, is_true,
+	Err, Result, at, debug_warn, is_true,
 	matrix::pdu::PduEvent,
 	result::FlatOk,
 	utils::{IterStream, stream::ReadyExt},
@@ -144,6 +144,17 @@ async fn category_room_events(
 		.map(at!(2))
 		.flatten()
 		.stream()
+		.then(|mut pdu| async {
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations to search result: {e}");
+			}
+			pdu
+		})
 		.map(PduEvent::into_room_event)
 		.map(|result| SearchResult {
 			rank: None,
diff --git a/src/api/client/sync/mod.rs b/src/api/client/sync/mod.rs
index db581bd6..1ea62883 100644
--- a/src/api/client/sync/mod.rs
+++ b/src/api/client/sync/mod.rs
@@ -3,7 +3,7 @@ mod v4;
 mod v5;
 
 use conduwuit::{
-	Error, PduCount, Result,
+	Error, PduCount, Result, debug_warn,
 	matrix::pdu::PduEvent,
 	utils::stream::{BroadbandExt, ReadyExt, TryIgnore},
 };
@@ -42,13 +42,23 @@ async fn load_timeline(
 		.timeline
 		.pdus_rev(room_id, None)
 		.ignore_err()
+		.ready_skip_while(|&(pducount, _)| pducount > next_batch.unwrap_or_else(PduCount::max))
+		.ready_take_while(|&(pducount, _)| pducount > roomsincecount)
 		.map(move |mut pdu| {
 			pdu.1.set_unsigned(Some(sender_user));
-			// TODO: bundled aggregations
 			pdu
 		})
-		.ready_skip_while(|&(pducount, _)| pducount > next_batch.unwrap_or_else(PduCount::max))
-		.ready_take_while(|&(pducount, _)| pducount > roomsincecount);
+		.then(async move |mut pdu| {
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(sender_user, &mut pdu.1)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations: {e}");
+			}
+			pdu
+		});
 
 	// Take the last events for the timeline
 	pin_mut!(non_timeline_pdus);
diff --git a/src/api/client/threads.rs b/src/api/client/threads.rs
index 404da4f8..09fb75d6 100644
--- a/src/api/client/threads.rs
+++ b/src/api/client/threads.rs
@@ -1,6 +1,6 @@
 use axum::extract::State;
 use conduwuit::{
-	Result, at,
+	Result, at, debug_warn,
 	matrix::pdu::{PduCount, PduEvent},
 };
 use futures::StreamExt;
@@ -28,7 +28,6 @@ pub(crate) async fn get_threads_route(
 		.transpose()?
 		.unwrap_or_else(PduCount::max);
 
-	// TODO: bundled aggregation
 	// TODO: user_can_see_event and set_unsigned should be at the same level /
 	// function, so unsigned is only set for seen events.
 	let threads: Vec<(PduCount, PduEvent)> = services
@@ -45,6 +44,17 @@ pub(crate) async fn get_threads_route(
 				.await
 				.then_some((count, pdu))
 		})
+		.then(|(count, mut pdu)| async move {
+			if let Err(e) = services
+				.rooms
+				.pdu_metadata
+				.add_bundled_aggregations_to_pdu(body.sender_user(), &mut pdu)
+				.await
+			{
+				debug_warn!("Failed to add bundled aggregations to thread: {e}");
+			}
+			(count, pdu)
+		})
 		.collect()
 		.await;
 
diff --git a/src/service/rooms/pdu_metadata/bundled_aggregations.rs b/src/service/rooms/pdu_metadata/bundled_aggregations.rs
new file mode 100644
index 00000000..4ef8efc1
--- /dev/null
+++ b/src/service/rooms/pdu_metadata/bundled_aggregations.rs
@@ -0,0 +1,394 @@
+use conduwuit::{Event, PduEvent, Result, err};
+use ruma::{
+	EventId, RoomId, UserId,
+	api::Direction,
+	events::relation::{BundledMessageLikeRelations, BundledReference, ReferenceChunk},
+};
+
+use super::PdusIterItem;
+
+const MAX_BUNDLED_RELATIONS: usize = 50;
+
+impl super::Service {
+	/// Gets bundled aggregations for an event according to the Matrix
+	/// specification.
+	/// - m.replace relations are bundled to include the most recent replacement
+	///   event.
+	/// - m.reference relations are bundled to include a chunk of event IDs.
+	#[tracing::instrument(skip(self), level = "debug")]
+	pub async fn get_bundled_aggregations(
+		&self,
+		user_id: &UserId,
+		room_id: &RoomId,
+		event_id: &EventId,
+	) -> Result<Option<BundledMessageLikeRelations<Box<serde_json::value::RawValue>>>> {
+		let relations = self
+			.get_relations(
+				user_id,
+				room_id,
+				event_id,
+				conduwuit::PduCount::max(),
+				MAX_BUNDLED_RELATIONS,
+				0,
+				Direction::Backward,
+			)
+			.await;
+		// The relations database code still handles the basic unsigned data
+		// We don't want to recursively fetch relations
+
+		// TODO: Event visibility check
+		// TODO: ignored users?
+
+		if relations.is_empty() {
+			return Ok(None);
+		}
+
+		let mut replace_events = Vec::with_capacity(relations.len().min(10)); // Most events have few replacements
+		let mut reference_events = Vec::with_capacity(relations.len());
+
+		for relation in &relations {
+			let pdu = &relation.1;
+
+			let content = pdu.get_content_as_value();
+			if let Some(relates_to) = content.get("m.relates_to") {
+				// We don't check that the event relates back, because we assume the database is
+				// good.
+				if let Some(rel_type) = relates_to.get("rel_type") {
+					match rel_type.as_str() {
+						| Some("m.replace") => {
+							replace_events.push(relation);
+						},
+						| Some("m.reference") => {
+							reference_events.push(relation);
+						},
+						| _ => {
+							// Ignore other relation types for now
+							// Threads are in the database but not handled here
+							// Other types are not specified AFAICT.
+						},
+					}
+				}
+			}
+		}
+
+		// If no relations to bundle, return None
+		if replace_events.is_empty() && reference_events.is_empty() {
+			return Ok(None);
+		}
+
+		let mut bundled = BundledMessageLikeRelations::new();
+
+		// Handle m.replace relations - find the most recent one
+		if !replace_events.is_empty() {
+			let most_recent_replacement = Self::find_most_recent_replacement(&replace_events)?;
+
+			// Convert the replacement event to the bundled format
+			if let Some(replacement_pdu) = most_recent_replacement {
+				// According to the Matrix spec, we should include the full event as raw JSON
+				let replacement_json = serde_json::to_string(replacement_pdu)
+					.map_err(|e| err!(Database("Failed to serialize replacement event: {e}")))?;
+				let raw_value = serde_json::value::RawValue::from_string(replacement_json)
+					.map_err(|e| err!(Database("Failed to create RawValue: {e}")))?;
+				bundled.replace = Some(Box::new(raw_value));
+			}
+		}
+
+		// Handle m.reference relations - collect event IDs
+		if !reference_events.is_empty() {
+			let reference_chunk = Self::build_reference_chunk(&reference_events)?;
+			if !reference_chunk.is_empty() {
+				bundled.reference = Some(Box::new(ReferenceChunk::new(reference_chunk)));
+			}
+		}
+
+		// TODO: Handle other relation types (m.annotation, etc.) when specified
+
+		Ok(Some(bundled))
+	}
+
+	/// Build reference chunk for m.reference bundled aggregations
+	fn build_reference_chunk(
+		reference_events: &[&PdusIterItem],
+	) -> Result<Vec<BundledReference>> {
+		let mut chunk = Vec::with_capacity(reference_events.len());
+
+		for relation in reference_events {
+			let pdu = &relation.1;
+
+			let reference_entry = BundledReference::new(pdu.event_id().to_owned());
+			chunk.push(reference_entry);
+		}
+
+		// Don't sort, order is unspecified
+
+		Ok(chunk)
+	}
+
+	/// Find the most recent replacement event based on origin_server_ts and
+	/// lexicographic event_id ordering
+	fn find_most_recent_replacement<'a>(
+		replacement_events: &'a [&'a PdusIterItem],
+	) -> Result<Option<&'a PduEvent>> {
+		if replacement_events.is_empty() {
+			return Ok(None);
+		}
+
+		let mut most_recent: Option<&PduEvent> = None;
+
+		// Jank, is there a better way to do this?
+		for relation in replacement_events {
+			let pdu = &relation.1;
+
+			match most_recent {
+				| None => {
+					most_recent = Some(pdu);
+				},
+				| Some(current_most_recent) => {
+					// Compare by origin_server_ts first
+					match pdu
+						.origin_server_ts()
+						.cmp(&current_most_recent.origin_server_ts())
+					{
+						| std::cmp::Ordering::Greater => {
+							most_recent = Some(pdu);
+						},
+						| std::cmp::Ordering::Equal => {
+							// If timestamps are equal, use lexicographic ordering of event_id
+							if pdu.event_id() > current_most_recent.event_id() {
+								most_recent = Some(pdu);
+							}
+						},
+						| std::cmp::Ordering::Less => {
+							// Keep current most recent
+						},
+					}
+				},
+			}
+		}
+
+		Ok(most_recent)
+	}
+
+	/// Adds bundled aggregations to a PDU's unsigned field
+	#[tracing::instrument(skip(self, pdu), level = "debug")]
+	pub async fn add_bundled_aggregations_to_pdu(
+		&self,
+		user_id: &UserId,
+		pdu: &mut PduEvent,
+	) -> Result<()> {
+		if pdu.is_redacted() {
+			return Ok(());
+		}
+
+		let bundled_aggregations = self
+			.get_bundled_aggregations(user_id, pdu.room_id(), pdu.event_id())
+			.await?;
+
+		if let Some(aggregations) = bundled_aggregations {
+			let aggregations_json = serde_json::to_value(aggregations)
+				.map_err(|e| err!(Database("Failed to serialize bundled aggregations: {e}")))?;
+
+			Self::add_bundled_aggregations_to_unsigned(pdu, aggregations_json)?;
+		}
+
+		Ok(())
+	}
+
+	/// Helper method to add bundled aggregations to a PDU's unsigned
+	/// field
+	fn add_bundled_aggregations_to_unsigned(
+		pdu: &mut PduEvent,
+		aggregations_json: serde_json::Value,
+	) -> Result<()> {
+		use serde_json::{
+			Map, Value as JsonValue,
+			value::{RawValue as RawJsonValue, to_raw_value},
+		};
+
+		let mut unsigned: Map<String, JsonValue> = pdu
+			.unsigned
+			.as_deref()
+			.map(RawJsonValue::get)
+			.map_or_else(|| Ok(Map::new()), serde_json::from_str)
+			.map_err(|e| err!(Database("Invalid unsigned in pdu event: {e}")))?;
+
+		let relations = unsigned
+			.entry("m.relations")
+			.or_insert_with(|| JsonValue::Object(Map::new()))
+			.as_object_mut()
+			.ok_or_else(|| err!(Database("m.relations is not an object")))?;
+
+		if let JsonValue::Object(aggregations_map) = aggregations_json {
+			for (rel_type, aggregation) in aggregations_map {
+				relations.insert(rel_type, aggregation);
+			}
+		}
+
+		pdu.unsigned = Some(to_raw_value(&unsigned)?);
+
+		Ok(())
+	}
+}
+
+#[cfg(test)]
+mod tests {
+	use conduwuit_core::pdu::{EventHash, PduEvent};
+	use ruma::{UInt, events::TimelineEventType, owned_event_id, owned_room_id, owned_user_id};
+	use serde_json::{Value as JsonValue, json, value::to_raw_value};
+
+	fn create_test_pdu(unsigned_content: Option<JsonValue>) -> PduEvent {
+		PduEvent {
+			event_id: owned_event_id!("$test:example.com"),
+			room_id: owned_room_id!("!test:example.com"),
+			sender: owned_user_id!("@test:example.com"),
+			origin_server_ts: UInt::try_from(1_234_567_890_u64).unwrap(),
+			kind: TimelineEventType::RoomMessage,
+			content: to_raw_value(&json!({"msgtype": "m.text", "body": "test"})).unwrap(),
+			state_key: None,
+			prev_events: vec![],
+			depth: UInt::from(1_u32),
+			auth_events: vec![],
+			redacts: None,
+			unsigned: unsigned_content.map(|content| to_raw_value(&content).unwrap()),
+			hashes: EventHash { sha256: "test_hash".to_owned() },
+			signatures: None,
+			origin: None,
+		}
+	}
+
+	fn create_bundled_aggregations() -> JsonValue {
+		json!({
+			"m.replace": {
+				"event_id": "$replace:example.com",
+				"origin_server_ts": 1_234_567_890,
+				"sender": "@replacer:example.com"
+			},
+			"m.reference": {
+				"count": 5,
+				"chunk": [
+					"$ref1:example.com",
+					"$ref2:example.com"
+				]
+			}
+		})
+	}
+
+	#[test]
+	fn test_add_bundled_aggregations_to_unsigned_no_existing_unsigned() {
+		let mut pdu = create_test_pdu(None);
+		let aggregations = create_bundled_aggregations();
+
+		let result = super::super::Service::add_bundled_aggregations_to_unsigned(
+			&mut pdu,
+			aggregations.clone(),
+		);
+		assert!(result.is_ok(), "Should succeed when no unsigned field exists");
+
+		assert!(pdu.unsigned.is_some(), "Unsigned field should be created");
+
+		let unsigned_str = pdu.unsigned.as_ref().unwrap().get();
+		let unsigned: JsonValue = serde_json::from_str(unsigned_str).unwrap();
+
+		assert!(unsigned.get("m.relations").is_some(), "m.relations should exist");
+		assert_eq!(
+			unsigned["m.relations"], aggregations,
+			"Relations should match the aggregations"
+		);
+	}
+
+	#[test]
+	fn test_add_bundled_aggregations_to_unsigned_overwrite_same_relation_type() {
+		let existing_unsigned = json!({
+			"m.relations": {
+				"m.replace": {
+					"event_id": "$old_replace:example.com",
+					"origin_server_ts": 1_111_111_111,
+					"sender": "@old_replacer:example.com"
+				}
+			}
+		});
+
+		let mut pdu = create_test_pdu(Some(existing_unsigned));
+		let new_aggregations = create_bundled_aggregations();
+
+		let result = super::super::Service::add_bundled_aggregations_to_unsigned(
+			&mut pdu,
+			new_aggregations.clone(),
+		);
+		assert!(result.is_ok(), "Should succeed when overwriting same relation type");
+
+		let unsigned_str = pdu.unsigned.as_ref().unwrap().get();
+		let unsigned: JsonValue = serde_json::from_str(unsigned_str).unwrap();
+
+		let relations = &unsigned["m.relations"];
+
+		assert_eq!(
+			relations["m.replace"], new_aggregations["m.replace"],
+			"m.replace should be updated"
+		);
+		assert_eq!(
+			relations["m.replace"]["event_id"], "$replace:example.com",
+			"Should have new event_id"
+		);
+
+		assert!(relations.get("m.reference").is_some(), "New m.reference should be added");
+	}
+
+	#[test]
+	fn test_add_bundled_aggregations_to_unsigned_preserve_other_unsigned_fields() {
+		// Test case: Other unsigned fields should be preserved
+		let existing_unsigned = json!({
+			"age": 98765,
+			"prev_content": {"msgtype": "m.text", "body": "old message"},
+			"redacted_because": {"event_id": "$redaction:example.com"},
+			"m.relations": {
+				"m.annotation": {"count": 1}
+			}
+		});
+
+		let mut pdu = create_test_pdu(Some(existing_unsigned));
+		let new_aggregations = json!({
+			"m.replace": {"event_id": "$new:example.com"}
+		});
+
+		let result = super::super::Service::add_bundled_aggregations_to_unsigned(
+			&mut pdu,
+			new_aggregations,
+		);
+		assert!(result.is_ok(), "Should succeed while preserving other fields");
+
+		let unsigned_str = pdu.unsigned.as_ref().unwrap().get();
+		let unsigned: JsonValue = serde_json::from_str(unsigned_str).unwrap();
+
+		// Verify all existing fields are preserved
+		assert_eq!(unsigned["age"], 98765, "age should be preserved");
+		assert!(unsigned.get("prev_content").is_some(), "prev_content should be preserved");
+		assert!(
+			unsigned.get("redacted_because").is_some(),
+			"redacted_because should be preserved"
+		);
+
+		// Verify relations were merged correctly
+		let relations = &unsigned["m.relations"];
+		assert!(
+			relations.get("m.annotation").is_some(),
+			"Existing m.annotation should be preserved"
+		);
+		assert!(relations.get("m.replace").is_some(), "New m.replace should be added");
+	}
+
+	#[test]
+	fn test_add_bundled_aggregations_to_unsigned_invalid_existing_unsigned() {
+		// Test case: Invalid JSON in existing unsigned should result in error
+		let mut pdu = create_test_pdu(None);
+		// Manually set invalid unsigned data
+		pdu.unsigned = Some(to_raw_value(&"invalid json").unwrap());
+
+		let aggregations = create_bundled_aggregations();
+		let result =
+			super::super::Service::add_bundled_aggregations_to_unsigned(&mut pdu, aggregations);
+
+		assert!(result.is_err(), "fails when existing unsigned is invalid");
+		// Should we ignore the error and overwrite anyway?
+	}
+}
diff --git a/src/service/rooms/pdu_metadata/mod.rs b/src/service/rooms/pdu_metadata/mod.rs
index 18221c2d..2dff54d8 100644
--- a/src/service/rooms/pdu_metadata/mod.rs
+++ b/src/service/rooms/pdu_metadata/mod.rs
@@ -1,3 +1,4 @@
+mod bundled_aggregations;
 mod data;
 use std::sync::Arc;