diff --git a/substrate/client/network/src/protocol.rs b/substrate/client/network/src/protocol.rs
index 357cd3baca..24d502ef2f 100644
--- a/substrate/client/network/src/protocol.rs
+++ b/substrate/client/network/src/protocol.rs
@@ -40,7 +40,7 @@ use sp_runtime::traits::{
 use sp_arithmetic::traits::SaturatedConversion;
 use message::{BlockAnnounce, Message};
 use message::generic::{Message as GenericMessage, ConsensusMessage};
-use prometheus_endpoint::{Registry, Gauge, GaugeVec, PrometheusError, Opts, register, U64};
+use prometheus_endpoint::{Registry, Gauge, GaugeVec, HistogramVec, PrometheusError, Opts, register, U64};
 use sync::{ChainSync, SyncState};
 use crate::service::{TransactionPool, ExHashT};
 use crate::config::{BoxFinalityProofRequestBuilder, Roles};
@@ -324,6 +324,7 @@ impl<B: BlockT, H: ExHashT> Protocol<B, H> {
 		block_announce_validator: Box<dyn BlockAnnounceValidator<B> + Send>,
 		metrics_registry: Option<&Registry>,
 		boot_node_ids: Arc<HashSet<PeerId>>,
+		queue_size_report: Option<HistogramVec>,
 	) -> error::Result<(Protocol<B, H>, sc_peerset::PeersetHandle)> {
 		let info = chain.info();
 		let sync = ChainSync::new(
@@ -346,7 +347,7 @@ impl<B: BlockT, H: ExHashT> Protocol<B, H> {
 
 		let (peerset, peerset_handle) = sc_peerset::Peerset::from_config(peerset_config);
 		let versions = &((MIN_VERSION as u8)..=(CURRENT_VERSION as u8)).collect::<Vec<u8>>();
-		let mut behaviour = GenericProto::new(protocol_id.clone(), versions, peerset);
+		let mut behaviour = GenericProto::new(protocol_id.clone(), versions, peerset, queue_size_report);
 
 		let mut legacy_equiv_by_name = HashMap::new();
 
@@ -2052,6 +2053,7 @@ mod tests {
 			Box::new(DefaultBlockAnnounceValidator::new(client.clone())),
 			None,
 			Default::default(),
+			None,
 		).unwrap();
 
 		let dummy_peer_id = PeerId::random();
diff --git a/substrate/client/network/src/protocol/generic_proto/behaviour.rs b/substrate/client/network/src/protocol/generic_proto/behaviour.rs
index e52ac5575f..c398f6df2d 100644
--- a/substrate/client/network/src/protocol/generic_proto/behaviour.rs
+++ b/substrate/client/network/src/protocol/generic_proto/behaviour.rs
@@ -24,6 +24,7 @@ use futures::prelude::*;
 use libp2p::core::{ConnectedPoint, Multiaddr, PeerId};
 use libp2p::swarm::{NetworkBehaviour, NetworkBehaviourAction, PollParameters};
 use log::{debug, error, trace, warn};
+use prometheus_endpoint::HistogramVec;
 use rand::distributions::{Distribution as _, Uniform};
 use smallvec::SmallVec;
 use std::task::{Context, Poll};
@@ -99,6 +100,9 @@ pub struct GenericProto {
 
 	/// Events to produce from `poll()`.
 	events: SmallVec<[NetworkBehaviourAction<NotifsHandlerIn, GenericProtoOut>; 4]>,
+
+	/// If `Some`, report the message queue sizes on this `Histogram`.
+	queue_size_report: Option<HistogramVec>,
 }
 
 /// State of a peer we're connected to.
@@ -267,10 +271,14 @@ pub enum GenericProtoOut {
 
 impl GenericProto {
 	/// Creates a `CustomProtos`.
+	///
+	/// The `queue_size_report` is an optional Prometheus metric that can report the size of the
+	/// messages queue. If passed, it must have one label for the protocol name.
 	pub fn new(
 		protocol: impl Into<ProtocolId>,
 		versions: &[u8],
 		peerset: sc_peerset::Peerset,
+		queue_size_report: Option<HistogramVec>,
 	) -> Self {
 		let legacy_protocol = RegisteredProtocol::new(protocol, versions);
 
@@ -282,6 +290,7 @@ impl GenericProto {
 			incoming: SmallVec::new(),
 			next_incoming_index: sc_peerset::IncomingIndex(0),
 			events: SmallVec::new(),
+			queue_size_report,
 		}
 	}
 
@@ -723,7 +732,11 @@ impl NetworkBehaviour for GenericProto {
 	type OutEvent = GenericProtoOut;
 
 	fn new_handler(&mut self) -> Self::ProtocolsHandler {
-		NotifsHandlerProto::new(self.legacy_protocol.clone(), self.notif_protocols.clone())
+		NotifsHandlerProto::new(
+			self.legacy_protocol.clone(),
+			self.notif_protocols.clone(),
+			self.queue_size_report.clone()
+		)
 	}
 
 	fn addresses_of_peer(&mut self, _: &PeerId) -> Vec<Multiaddr> {
diff --git a/substrate/client/network/src/protocol/generic_proto/handler/group.rs b/substrate/client/network/src/protocol/generic_proto/handler/group.rs
index 6b23263b14..21dc4091c0 100644
--- a/substrate/client/network/src/protocol/generic_proto/handler/group.rs
+++ b/substrate/client/network/src/protocol/generic_proto/handler/group.rs
@@ -64,6 +64,7 @@ use libp2p::swarm::{
 	NegotiatedSubstream,
 };
 use log::{debug, error};
+use prometheus_endpoint::HistogramVec;
 use std::{borrow::Cow, error, io, str, task::{Context, Poll}};
 
 /// Implements the `IntoProtocolsHandler` trait of libp2p.
@@ -225,12 +226,30 @@ pub enum NotifsHandlerOut {
 
 impl NotifsHandlerProto {
 	/// Builds a new handler.
-	pub fn new(legacy: RegisteredProtocol, list: impl Into<Vec<(Cow<'static, [u8]>, Vec<u8>)>>) -> Self {
+	///
+	/// The `queue_size_report` is an optional Prometheus metric that can report the size of the
+	/// messages queue. If passed, it must have one label for the protocol name.
+	pub fn new(legacy: RegisteredProtocol, list: impl Into<Vec<(Cow<'static, [u8]>, Vec<u8>)>>, queue_size_report: Option<HistogramVec>) -> Self {
 		let list = list.into();
 
+		let out_handlers = list
+			.clone()
+			.into_iter()
+			.map(|(p, _)| {
+				let queue_size_report = queue_size_report.as_ref().and_then(|qs| {
+					if let Ok(utf8) = str::from_utf8(&p) {
+						Some(qs.with_label_values(&[utf8]))
+					} else {
+						log::warn!("Ignoring Prometheus metric because {:?} isn't UTF-8", p);
+						None
+					}
+				});
+				NotifsOutHandlerProto::new(p, queue_size_report)
+			}).collect();
+
 		NotifsHandlerProto {
 			in_handlers: list.clone().into_iter().map(|(p, _)| NotifsInHandlerProto::new(p)).collect(),
-			out_handlers: list.clone().into_iter().map(|(p, _)| NotifsOutHandlerProto::new(p)).collect(),
+			out_handlers,
 			legacy: LegacyProtoHandlerProto::new(legacy),
 		}
 	}
diff --git a/substrate/client/network/src/protocol/generic_proto/handler/notif_out.rs b/substrate/client/network/src/protocol/generic_proto/handler/notif_out.rs
index 62d2409be8..63b460bf8c 100644
--- a/substrate/client/network/src/protocol/generic_proto/handler/notif_out.rs
+++ b/substrate/client/network/src/protocol/generic_proto/handler/notif_out.rs
@@ -34,6 +34,7 @@ use libp2p::swarm::{
 	NegotiatedSubstream,
 };
 use log::error;
+use prometheus_endpoint::Histogram;
 use smallvec::SmallVec;
 use std::{borrow::Cow, fmt, mem, pin::Pin, task::{Context, Poll}, time::Duration};
 use wasm_timer::Instant;
@@ -56,14 +57,17 @@ const INITIAL_KEEPALIVE_TIME: Duration = Duration::from_secs(5);
 pub struct NotifsOutHandlerProto {
 	/// Name of the protocol to negotiate.
 	protocol_name: Cow<'static, [u8]>,
+	/// Optional Prometheus histogram to report message queue size variations.
+	queue_size_report: Option<Histogram>,
 }
 
 impl NotifsOutHandlerProto {
 	/// Builds a new [`NotifsOutHandlerProto`]. Will use the given protocol name for the
 	/// notifications substream.
-	pub fn new(protocol_name: impl Into<Cow<'static, [u8]>>) -> Self {
+	pub fn new(protocol_name: impl Into<Cow<'static, [u8]>>, queue_size_report: Option<Histogram>) -> Self {
 		NotifsOutHandlerProto {
 			protocol_name: protocol_name.into(),
+			queue_size_report,
 		}
 	}
 }
@@ -79,6 +83,7 @@ impl IntoProtocolsHandler for NotifsOutHandlerProto {
 		NotifsOutHandler {
 			protocol_name: self.protocol_name,
 			when_connection_open: Instant::now(),
+			queue_size_report: self.queue_size_report,
 			state: State::Disabled,
 			events_queue: SmallVec::new(),
 		}
@@ -103,6 +108,9 @@ pub struct NotifsOutHandler {
 	/// When the connection with the remote has been successfully established.
 	when_connection_open: Instant,
 
+	/// Optional prometheus histogram to report message queue sizes variations.
+	queue_size_report: Option<Histogram>,
+
 	/// Queue of events to send to the outside.
 	///
 	/// This queue must only ever be modified to insert elements at the back, or remove the first
@@ -301,6 +309,9 @@ impl ProtocolsHandler for NotifsOutHandler {
 			NotifsOutHandlerIn::Send(msg) =>
 				if let State::Open { substream, .. } = &mut self.state {
 					if let Some(Ok(_)) = substream.send(msg).now_or_never() {
+						if let Some(metric) = &self.queue_size_report {
+							metric.observe(substream.queue_len() as f64);
+						}
 					} else {
 						log::warn!(
 							target: "sub-libp2p",
diff --git a/substrate/client/network/src/protocol/generic_proto/tests.rs b/substrate/client/network/src/protocol/generic_proto/tests.rs
index 00b840d581..c0582d5f5c 100644
--- a/substrate/client/network/src/protocol/generic_proto/tests.rs
+++ b/substrate/client/network/src/protocol/generic_proto/tests.rs
@@ -82,7 +82,7 @@ fn build_nodes() -> (Swarm<CustomProtoWithAddr>, Swarm<CustomProtoWithAddr>) {
 		});
 
 		let behaviour = CustomProtoWithAddr {
-			inner: GenericProto::new(&b"test"[..], &[1], peerset),
+			inner: GenericProto::new(&b"test"[..], &[1], peerset, None),
 			addrs: addrs
 				.iter()
 				.enumerate()
diff --git a/substrate/client/network/src/protocol/generic_proto/upgrade/notifications.rs b/substrate/client/network/src/protocol/generic_proto/upgrade/notifications.rs
index b6ae1425f1..de42d85c80 100644
--- a/substrate/client/network/src/protocol/generic_proto/upgrade/notifications.rs
+++ b/substrate/client/network/src/protocol/generic_proto/upgrade/notifications.rs
@@ -38,7 +38,7 @@ use futures::{prelude::*, ready};
 use futures_codec::Framed;
 use libp2p::core::{UpgradeInfo, InboundUpgrade, OutboundUpgrade, upgrade};
 use log::error;
-use std::{borrow::Cow, collections::VecDeque, io, iter, mem, pin::Pin, task::{Context, Poll}};
+use std::{borrow::Cow, collections::VecDeque, convert::TryFrom as _, io, iter, mem, pin::Pin, task::{Context, Poll}};
 use unsigned_varint::codec::UviBytes;
 
 /// Maximum allowed size of the two handshake messages, in bytes.
@@ -280,6 +280,13 @@ where TSubstream: AsyncRead + AsyncWrite + Unpin + Send + 'static,
 	}
 }
 
+impl<TSubstream> NotificationsOutSubstream<TSubstream> {
+	/// Returns the number of items in the queue, capped to `u32::max_value()`.
+	pub fn queue_len(&self) -> u32 {
+		u32::try_from(self.messages_queue.len()).unwrap_or(u32::max_value())
+	}
+}
+
 impl<TSubstream> Sink<Vec<u8>> for NotificationsOutSubstream<TSubstream>
 	where TSubstream: AsyncRead + AsyncWrite + Unpin,
 {
diff --git a/substrate/client/network/src/service.rs b/substrate/client/network/src/service.rs
index de22f203bc..8d2f0e033f 100644
--- a/substrate/client/network/src/service.rs
+++ b/substrate/client/network/src/service.rs
@@ -42,7 +42,7 @@ use libp2p::{kad::record, Multiaddr, PeerId};
 use log::{error, info, trace, warn};
 use parking_lot::Mutex;
 use prometheus_endpoint::{
-	register, Counter, CounterVec, Gauge, GaugeVec, Opts, PrometheusError, Registry, U64,
+	register, Counter, CounterVec, Gauge, GaugeVec, HistogramOpts, HistogramVec, Opts, PrometheusError, Registry, U64,
 };
 use sc_peerset::PeersetHandle;
 use sp_consensus::import_queue::{BlockImportError, BlockImportResult, ImportQueue, Link};
@@ -239,6 +239,12 @@ impl<B: BlockT + 'static, H: ExHashT> NetworkWorker<B, H> {
 		let local_peer_id = local_public.clone().into_peer_id();
 		info!(target: "sub-libp2p", "🏷  Local node identity is: {}", local_peer_id.to_base58());
 
+		// Initialize the metrics.
+		let metrics = match &params.metrics_registry {
+			Some(registry) => Some(Metrics::register(&registry)?),
+			None => None
+		};
+
 		let checker = params.on_demand.as_ref()
 			.map(|od| od.checker().clone())
 			.unwrap_or(Arc::new(AlwaysBadChecker));
@@ -259,6 +265,7 @@ impl<B: BlockT + 'static, H: ExHashT> NetworkWorker<B, H> {
 			params.block_announce_validator,
 			params.metrics_registry.as_ref(),
 			boot_node_ids.clone(),
+			metrics.as_ref().map(|m| m.notifications_queues_size.clone()),
 		)?;
 
 		// Build the swarm.
@@ -343,10 +350,7 @@ impl<B: BlockT + 'static, H: ExHashT> NetworkWorker<B, H> {
 			from_worker,
 			light_client_rqs: params.on_demand.and_then(|od| od.extract_receiver()),
 			event_streams: Vec::new(),
-			metrics: match params.metrics_registry {
-				Some(registry) => Some(Metrics::register(&registry)?),
-				None => None
-			},
+			metrics,
 			boot_node_ids,
 		})
 	}
@@ -802,6 +806,7 @@ struct Metrics {
 	issued_light_requests: Counter<U64>,
 	kbuckets_num_nodes: Gauge<U64>,
 	network_per_sec_bytes: GaugeVec<U64>,
+	notifications_queues_size: HistogramVec,
 	notifications_total: CounterVec<U64>,
 	num_event_stream_channels: Gauge<U64>,
 	opened_notification_streams: GaugeVec<U64>,
@@ -847,6 +852,16 @@ impl Metrics {
 				),
 				&["direction"]
 			)?, registry)?,
+			notifications_queues_size: register(HistogramVec::new(
+				HistogramOpts {
+					common_opts: Opts::new(
+						"sub_libp2p_notifications_queues_size",
+						"Total size of all the notification queues"
+					),
+					buckets: vec![0.0, 1.0, 2.0, 4.0, 8.0, 16.0, 32.0, 64.0, 128.0, 256.0],
+				},
+				&["protocol"]
+			)?, registry)?,
 			notifications_total: register(CounterVec::new(
 				Opts::new(
 					"sub_libp2p_notifications_total",
diff --git a/substrate/utils/prometheus/src/lib.rs b/substrate/utils/prometheus/src/lib.rs
index 4a8f05d929..9dab2a2695 100644
--- a/substrate/utils/prometheus/src/lib.rs
+++ b/substrate/utils/prometheus/src/lib.rs
@@ -17,6 +17,7 @@
 use futures_util::{FutureExt, future::Future};
 pub use prometheus::{
 	Registry, Error as PrometheusError, Opts,
+	Histogram, HistogramOpts, HistogramVec,
 	core::{
 		GenericGauge as Gauge, GenericCounter as Counter,
 		GenericGaugeVec as GaugeVec, GenericCounterVec as CounterVec,