diff --git a/substrate/bin/node-template/runtime/src/lib.rs b/substrate/bin/node-template/runtime/src/lib.rs
index 63d79e6047..eae40e1ab3 100644
--- a/substrate/bin/node-template/runtime/src/lib.rs
+++ b/substrate/bin/node-template/runtime/src/lib.rs
@@ -195,9 +195,14 @@ impl frame_system::Config for Runtime {
 
 impl pallet_randomness_collective_flip::Config for Runtime {}
 
+parameter_types! {
+	pub const MaxAuthorities: u32 = 32;
+}
+
 impl pallet_aura::Config for Runtime {
 	type AuthorityId = AuraId;
 	type DisabledValidators = ();
+	type MaxAuthorities = MaxAuthorities;
 }
 
 impl pallet_grandpa::Config for Runtime {
@@ -382,7 +387,7 @@ impl_runtime_apis! {
 		}
 
 		fn authorities() -> Vec<AuraId> {
-			Aura::authorities()
+			Aura::authorities().into_inner()
 		}
 	}
 
diff --git a/substrate/frame/aura/Cargo.toml b/substrate/frame/aura/Cargo.toml
index f6aa4fac2f..ee7b15f91e 100644
--- a/substrate/frame/aura/Cargo.toml
+++ b/substrate/frame/aura/Cargo.toml
@@ -14,9 +14,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
 sp-application-crypto = { version = "4.0.0-dev", default-features = false, path = "../../primitives/application-crypto" }
-codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false, features = [
-	"derive",
-] }
+codec = { package = "parity-scale-codec", version = "2.2.0", default-features = false, features = ["derive", "max-encoded-len"] }
 sp-std = { version = "4.0.0-dev", default-features = false, path = "../../primitives/std" }
 sp-runtime = { version = "4.0.0-dev", default-features = false, path = "../../primitives/runtime" }
 frame-support = { version = "4.0.0-dev", default-features = false, path = "../support" }
diff --git a/substrate/frame/aura/src/lib.rs b/substrate/frame/aura/src/lib.rs
index 1138a3e850..e8b68f928e 100644
--- a/substrate/frame/aura/src/lib.rs
+++ b/substrate/frame/aura/src/lib.rs
@@ -38,10 +38,10 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use codec::{Decode, Encode};
+use codec::{Decode, Encode, MaxEncodedLen};
 use frame_support::{
 	traits::{DisabledValidators, FindAuthor, Get, OnTimestampSet, OneSessionHandler},
-	ConsensusEngineId, Parameter,
+	BoundedSlice, ConsensusEngineId, Parameter, WeakBoundedVec,
 };
 use sp_consensus_aura::{AuthorityIndex, ConsensusLog, Slot, AURA_ENGINE_ID};
 use sp_runtime::{
@@ -49,7 +49,7 @@ use sp_runtime::{
 	traits::{IsMember, Member, SaturatedConversion, Saturating, Zero},
 	RuntimeAppPublic,
 };
-use sp_std::prelude::*;
+use sp_std::{convert::TryFrom, vec::Vec};
 
 pub mod migrations;
 mod mock;
@@ -70,7 +70,10 @@ pub mod pallet {
 			+ Parameter
 			+ RuntimeAppPublic
 			+ Default
-			+ MaybeSerializeDeserialize;
+			+ MaybeSerializeDeserialize
+			+ MaxEncodedLen;
+		/// The maximum number of authorities that the pallet can hold.
+		type MaxAuthorities: Get<u32>;
 
 		/// A way to check whether a given validator is disabled and should not be authoring blocks.
 		/// Blocks authored by a disabled validator will lead to a panic as part of this module's
@@ -79,6 +82,7 @@ pub mod pallet {
 	}
 
 	#[pallet::pallet]
+	#[pallet::generate_storage_info]
 	pub struct Pallet<T>(sp_std::marker::PhantomData<T>);
 
 	#[pallet::hooks]
@@ -113,7 +117,8 @@ pub mod pallet {
 	/// The current authority set.
 	#[pallet::storage]
 	#[pallet::getter(fn authorities)]
-	pub(super) type Authorities<T: Config> = StorageValue<_, Vec<T::AuthorityId>, ValueQuery>;
+	pub(super) type Authorities<T: Config> =
+		StorageValue<_, WeakBoundedVec<T::AuthorityId, T::MaxAuthorities>, ValueQuery>;
 
 	/// The current slot of this block.
 	///
@@ -143,18 +148,22 @@ pub mod pallet {
 }
 
 impl<T: Config> Pallet<T> {
-	fn change_authorities(new: Vec<T::AuthorityId>) {
+	fn change_authorities(new: WeakBoundedVec<T::AuthorityId, T::MaxAuthorities>) {
 		<Authorities<T>>::put(&new);
 
-		let log: DigestItem<T::Hash> =
-			DigestItem::Consensus(AURA_ENGINE_ID, ConsensusLog::AuthoritiesChange(new).encode());
+		let log: DigestItem<T::Hash> = DigestItem::Consensus(
+			AURA_ENGINE_ID,
+			ConsensusLog::AuthoritiesChange(new.into_inner()).encode(),
+		);
 		<frame_system::Pallet<T>>::deposit_log(log.into());
 	}
 
 	fn initialize_authorities(authorities: &[T::AuthorityId]) {
 		if !authorities.is_empty() {
 			assert!(<Authorities<T>>::get().is_empty(), "Authorities are already initialized!");
-			<Authorities<T>>::put(authorities);
+			let bounded = <BoundedSlice<'_, _, T::MaxAuthorities>>::try_from(authorities)
+				.expect("Initial authority set must be less than T::MaxAuthorities");
+			<Authorities<T>>::put(bounded);
 		}
 	}
 
@@ -202,8 +211,12 @@ impl<T: Config> OneSessionHandler<T::AccountId> for Pallet<T> {
 		if changed {
 			let next_authorities = validators.map(|(_, k)| k).collect::<Vec<_>>();
 			let last_authorities = Self::authorities();
-			if next_authorities != last_authorities {
-				Self::change_authorities(next_authorities);
+			if last_authorities != next_authorities {
+				let bounded = <WeakBoundedVec<_, T::MaxAuthorities>>::force_from(
+					next_authorities,
+					Some("AuRa new session"),
+				);
+				Self::change_authorities(bounded);
 			}
 		}
 	}
diff --git a/substrate/frame/aura/src/mock.rs b/substrate/frame/aura/src/mock.rs
index 0e258fb9a6..4418d9e85a 100644
--- a/substrate/frame/aura/src/mock.rs
+++ b/substrate/frame/aura/src/mock.rs
@@ -87,6 +87,10 @@ impl pallet_timestamp::Config for Test {
 	type WeightInfo = ();
 }
 
+parameter_types! {
+	pub const MaxAuthorities: u32 = 10;
+}
+
 thread_local! {
 	static DISABLED_VALIDATORS: RefCell<Vec<AuthorityIndex>> = RefCell::new(Default::default());
 }
@@ -113,6 +117,7 @@ impl DisabledValidators for MockDisabledValidators {
 impl pallet_aura::Config for Test {
 	type AuthorityId = AuthorityId;
 	type DisabledValidators = MockDisabledValidators;
+	type MaxAuthorities = MaxAuthorities;
 }
 
 pub fn new_test_ext(authorities: Vec<u64>) -> sp_io::TestExternalities {
diff --git a/substrate/frame/support/src/storage/bounded_vec.rs b/substrate/frame/support/src/storage/bounded_vec.rs
index 6d25e058c0..0f56511e6e 100644
--- a/substrate/frame/support/src/storage/bounded_vec.rs
+++ b/substrate/frame/support/src/storage/bounded_vec.rs
@@ -21,6 +21,7 @@
 use crate::{
 	storage::{StorageDecodeLength, StorageTryAppend},
 	traits::Get,
+	WeakBoundedVec,
 };
 use codec::{Decode, Encode, EncodeLike, MaxEncodedLen};
 use core::{
@@ -45,8 +46,13 @@ pub struct BoundedVec<T, S>(Vec<T>, PhantomData<S>);
 #[derive(Encode)]
 pub struct BoundedSlice<'a, T, S>(&'a [T], PhantomData<S>);
 
-// `BoundedSlice`s encode to something which will always decode into a `BoundedVec` or a `Vec`.
+// `BoundedSlice`s encode to something which will always decode into a `BoundedVec`,
+// `WeakBoundedVec`, or a `Vec`.
 impl<'a, T: Encode + Decode, S: Get<u32>> EncodeLike<BoundedVec<T, S>> for BoundedSlice<'a, T, S> {}
+impl<'a, T: Encode + Decode, S: Get<u32>> EncodeLike<WeakBoundedVec<T, S>>
+	for BoundedSlice<'a, T, S>
+{
+}
 impl<'a, T: Encode + Decode, S: Get<u32>> EncodeLike<Vec<T>> for BoundedSlice<'a, T, S> {}
 
 impl<'a, T, S: Get<u32>> TryFrom<&'a [T]> for BoundedSlice<'a, T, S> {
diff --git a/substrate/primitives/consensus/slots/Cargo.toml b/substrate/primitives/consensus/slots/Cargo.toml
index 9619f627a0..2718158cfb 100644
--- a/substrate/primitives/consensus/slots/Cargo.toml
+++ b/substrate/primitives/consensus/slots/Cargo.toml
@@ -13,7 +13,7 @@ readme = "README.md"
 targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
-codec = { package = "parity-scale-codec", version = "2.0.0", default-features = false, features = ["derive"] }
+codec = { package = "parity-scale-codec", version = "2.2.0", default-features = false, features = ["derive", "max-encoded-len"] }
 sp-runtime = { version = "4.0.0-dev", default-features = false, path = "../../runtime" }
 sp-arithmetic = { version = "4.0.0-dev", default-features = false, path = "../../arithmetic" }