diff --git a/core/src/protocols_handler/dummy.rs b/core/src/protocols_handler/dummy.rs
index 113c31c9..f400fddc 100644
--- a/core/src/protocols_handler/dummy.rs
+++ b/core/src/protocols_handler/dummy.rs
@@ -27,7 +27,7 @@ use crate::{
     }
 };
 use futures::prelude::*;
-use std::{io, marker::PhantomData};
+use std::marker::PhantomData;
 use tokio_io::{AsyncRead, AsyncWrite};
 use void::Void;
 
@@ -53,6 +53,7 @@ where
 {
     type InEvent = Void;
     type OutEvent = Void;
+    type Error = Void;
     type Substream = TSubstream;
     type InboundProtocol = DeniedUpgrade;
     type OutboundProtocol = DeniedUpgrade;
@@ -97,7 +98,7 @@ where
         &mut self,
     ) -> Poll<
         Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>,
-        io::Error,
+        Void,
     > {
         if self.shutting_down {
             Ok(Async::Ready(None))
diff --git a/core/src/protocols_handler/map_in.rs b/core/src/protocols_handler/map_in.rs
index 93b97810..3a4861a8 100644
--- a/core/src/protocols_handler/map_in.rs
+++ b/core/src/protocols_handler/map_in.rs
@@ -26,7 +26,7 @@ use crate::{
     }
 };
 use futures::prelude::*;
-use std::{io, marker::PhantomData};
+use std::marker::PhantomData;
 
 /// Wrapper around a protocol handler that turns the input event into something else.
 pub struct MapInEvent<TProtoHandler, TNewIn, TMap> {
@@ -54,6 +54,7 @@ where
 {
     type InEvent = TNewIn;
     type OutEvent = TProtoHandler::OutEvent;
+    type Error = TProtoHandler::Error;
     type Substream = TProtoHandler::Substream;
     type InboundProtocol = TProtoHandler::InboundProtocol;
     type OutboundProtocol = TProtoHandler::OutboundProtocol;
@@ -108,7 +109,7 @@ where
         &mut self,
     ) -> Poll<
         Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>,
-        io::Error,
+        Self::Error,
     > {
         self.inner.poll()
     }
diff --git a/core/src/protocols_handler/map_out.rs b/core/src/protocols_handler/map_out.rs
index e7c3e6d9..98103fc2 100644
--- a/core/src/protocols_handler/map_out.rs
+++ b/core/src/protocols_handler/map_out.rs
@@ -26,7 +26,6 @@ use crate::{
     }
 };
 use futures::prelude::*;
-use std::io;
 
 /// Wrapper around a protocol handler that turns the output event into something else.
 pub struct MapOutEvent<TProtoHandler, TMap> {
@@ -52,6 +51,7 @@ where
 {
     type InEvent = TProtoHandler::InEvent;
     type OutEvent = TNewOut;
+    type Error = TProtoHandler::Error;
     type Substream = TProtoHandler::Substream;
     type InboundProtocol = TProtoHandler::InboundProtocol;
     type OutboundProtocol = TProtoHandler::OutboundProtocol;
@@ -104,7 +104,7 @@ where
         &mut self,
     ) -> Poll<
         Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>,
-        io::Error,
+        Self::Error,
     > {
         Ok(self.inner.poll()?.map(|ev| {
             ev.map(|ev| match ev {
diff --git a/core/src/protocols_handler/mod.rs b/core/src/protocols_handler/mod.rs
index a8b7690c..2889141d 100644
--- a/core/src/protocols_handler/mod.rs
+++ b/core/src/protocols_handler/mod.rs
@@ -24,7 +24,7 @@ use crate::upgrade::{
     UpgradeError,
 };
 use futures::prelude::*;
-use std::{error, fmt, io, time::Duration};
+use std::{error, fmt, time::Duration};
 use tokio_io::{AsyncRead, AsyncWrite};
 
 pub use self::dummy::DummyProtocolsHandler;
@@ -89,6 +89,8 @@ pub trait ProtocolsHandler {
     type InEvent;
     /// Custom event that can be produced by the handler and that will be returned to the outside.
     type OutEvent;
+    /// Error that can happen when polling.
+    type Error: error::Error;
     /// The type of the substream that contains the raw data.
     type Substream: AsyncRead + AsyncWrite;
     /// The upgrade for the protocol or protocols handled by this handler.
@@ -144,7 +146,7 @@ pub trait ProtocolsHandler {
     /// > **Note**: If this handler is combined with other handlers, as soon as `poll()` returns
     /// >           `Ok(Async::Ready(None))`, all the other handlers will receive a call to
     /// >           `shutdown()` and will eventually be closed and destroyed.
-    fn poll(&mut self) -> Poll<Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>, io::Error>;
+    fn poll(&mut self) -> Poll<Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>, Self::Error>;
 
     /// Adds a closure that turns the input event into something else.
     #[inline]
diff --git a/core/src/protocols_handler/node_handler.rs b/core/src/protocols_handler/node_handler.rs
index af38b841..8f709090 100644
--- a/core/src/protocols_handler/node_handler.rs
+++ b/core/src/protocols_handler/node_handler.rs
@@ -29,7 +29,7 @@ use crate::{
     }
 };
 use futures::prelude::*;
-use std::{io, time::Duration};
+use std::time::Duration;
 use tokio_timer::Timeout;
 
 /// Prototype for a `NodeHandlerWrapper`.
@@ -123,7 +123,7 @@ where
 {
     type InEvent = TProtoHandler::InEvent;
     type OutEvent = TProtoHandler::OutEvent;
-    type Error = io::Error;       // TODO: better error type
+    type Error = TProtoHandler::Error;
     type Substream = TProtoHandler::Substream;
     // The first element of the tuple is the unique upgrade identifier
     // (see `unique_dial_upgrade_id`).
@@ -198,7 +198,7 @@ where
         self.handler.shutdown();
     }
 
-    fn poll(&mut self) -> Poll<Option<NodeHandlerEvent<Self::OutboundOpenInfo, Self::OutEvent>>, io::Error> {
+    fn poll(&mut self) -> Poll<Option<NodeHandlerEvent<Self::OutboundOpenInfo, Self::OutEvent>>, Self::Error> {
         // Continue negotiation of newly-opened substreams on the listening side.
         // We remove each element from `negotiating_in` one by one and add them back if not ready.
         for n in (0..self.negotiating_in.len()).rev() {
diff --git a/core/src/protocols_handler/select.rs b/core/src/protocols_handler/select.rs
index 196ef411..86a27f8d 100644
--- a/core/src/protocols_handler/select.rs
+++ b/core/src/protocols_handler/select.rs
@@ -31,7 +31,6 @@ use crate::{
     }
 };
 use futures::prelude::*;
-use std::io;
 use tokio_io::{AsyncRead, AsyncWrite};
 
 /// Implementation of `ProtocolsHandler` that combines two protocols into one.
@@ -65,6 +64,7 @@ where
 {
     type InEvent = EitherOutput<TProto1::InEvent, TProto2::InEvent>;
     type OutEvent = EitherOutput<TProto1::OutEvent, TProto2::OutEvent>;
+    type Error = EitherError<TProto1::Error, TProto2::Error>;
     type Substream = TSubstream;
     type InboundProtocol = SelectUpgrade<TProto1::InboundProtocol, TProto2::InboundProtocol>;
     type OutboundProtocol = EitherUpgrade<TProto1::OutboundProtocol, TProto2::OutboundProtocol>;
@@ -161,8 +161,8 @@ where
         self.proto2.shutdown();
     }
 
-    fn poll(&mut self) -> Poll<Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>, io::Error> {
-        match self.proto1.poll()? {
+    fn poll(&mut self) -> Poll<Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>, Self::Error> {
+        match self.proto1.poll().map_err(EitherError::A)? {
             Async::Ready(Some(ProtocolsHandlerEvent::Custom(event))) => {
                 return Ok(Async::Ready(Some(ProtocolsHandlerEvent::Custom(EitherOutput::First(event)))));
             },
@@ -176,7 +176,7 @@ where
             Async::NotReady => ()
         };
 
-        match self.proto2.poll()? {
+        match self.proto2.poll().map_err(EitherError::B)? {
             Async::Ready(Some(ProtocolsHandlerEvent::Custom(event))) => {
                 return Ok(Async::Ready(Some(ProtocolsHandlerEvent::Custom(EitherOutput::Second(event)))));
             },
diff --git a/core/src/swarm.rs b/core/src/swarm.rs
index 02c99b7e..24b76dee 100644
--- a/core/src/swarm.rs
+++ b/core/src/swarm.rs
@@ -45,7 +45,7 @@ where TTransport: Transport,
         <<TBehaviour as NetworkBehaviour<TTopology>>::ProtocolsHandler as ProtocolsHandler>::InEvent,
         <<TBehaviour as NetworkBehaviour<TTopology>>::ProtocolsHandler as ProtocolsHandler>::OutEvent,
         NodeHandlerWrapper<TBehaviour::ProtocolsHandler>,
-        io::Error,
+        <<TBehaviour as NetworkBehaviour<TTopology>>::ProtocolsHandler as ProtocolsHandler>::Error,
     >,
 
     /// Handles which nodes to connect to and how to handle the events sent back by the protocol
@@ -97,6 +97,7 @@ where TBehaviour: NetworkBehaviour<TTopology>,
       TBehaviour::ProtocolsHandler: ProtocolsHandler<Substream = Substream<TMuxer>> + Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::InEvent: Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::OutEvent: Send + 'static,
+      <TBehaviour::ProtocolsHandler as ProtocolsHandler>::Error: Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::OutboundOpenInfo: Send + 'static, // TODO: shouldn't be necessary
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::InboundProtocol: InboundUpgrade<Substream<TMuxer>> + Send + 'static,
       <<TBehaviour::ProtocolsHandler as ProtocolsHandler>::InboundProtocol as UpgradeInfo>::Info: Send + 'static,
@@ -213,6 +214,7 @@ where TBehaviour: NetworkBehaviour<TTopology>,
       TBehaviour::ProtocolsHandler: ProtocolsHandler<Substream = Substream<TMuxer>> + Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::InEvent: Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::OutEvent: Send + 'static,
+      <TBehaviour::ProtocolsHandler as ProtocolsHandler>::Error: Send + 'static,
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::OutboundOpenInfo: Send + 'static, // TODO: shouldn't be necessary
       <TBehaviour::ProtocolsHandler as ProtocolsHandler>::InboundProtocol: InboundUpgrade<Substream<TMuxer>> + Send + 'static,
       <<TBehaviour::ProtocolsHandler as ProtocolsHandler>::InboundProtocol as InboundUpgrade<Substream<TMuxer>>>::Future: Send + 'static,
diff --git a/protocols/floodsub/src/handler.rs b/protocols/floodsub/src/handler.rs
index 0b663a39..c084b8cf 100644
--- a/protocols/floodsub/src/handler.rs
+++ b/protocols/floodsub/src/handler.rs
@@ -105,6 +105,7 @@ where
 {
     type InEvent = FloodsubRpc;
     type OutEvent = FloodsubRpc;
+    type Error = io::Error;
     type Substream = TSubstream;
     type InboundProtocol = FloodsubConfig;
     type OutboundProtocol = FloodsubConfig;
diff --git a/protocols/identify/src/listen_handler.rs b/protocols/identify/src/listen_handler.rs
index a7f9982a..650b4780 100644
--- a/protocols/identify/src/listen_handler.rs
+++ b/protocols/identify/src/listen_handler.rs
@@ -25,7 +25,6 @@ use libp2p_core::{
     upgrade::{DeniedUpgrade, InboundUpgrade, OutboundUpgrade}
 };
 use smallvec::SmallVec;
-use std::io;
 use tokio_io::{AsyncRead, AsyncWrite};
 use void::{Void, unreachable};
 
@@ -59,6 +58,7 @@ where
 {
     type InEvent = Void;
     type OutEvent = IdentifySender<TSubstream>;
+    type Error = Void;
     type Substream = TSubstream;
     type InboundProtocol = IdentifyProtocolConfig;
     type OutboundProtocol = DeniedUpgrade;
@@ -104,7 +104,7 @@ where
                 Self::OutEvent,
             >,
         >,
-        io::Error,
+        Self::Error,
     > {
         if !self.pending_result.is_empty() {
             return Ok(Async::Ready(Some(ProtocolsHandlerEvent::Custom(
diff --git a/protocols/identify/src/periodic_id_handler.rs b/protocols/identify/src/periodic_id_handler.rs
index b76036af..ef0dd500 100644
--- a/protocols/identify/src/periodic_id_handler.rs
+++ b/protocols/identify/src/periodic_id_handler.rs
@@ -26,7 +26,7 @@ use libp2p_core::{
 };
 use std::{io, marker::PhantomData, time::{Duration, Instant}};
 use tokio_io::{AsyncRead, AsyncWrite};
-use tokio_timer::Delay;
+use tokio_timer::{self, Delay};
 use void::{Void, unreachable};
 
 /// Delay between the moment we connect and the first time we identify.
@@ -81,6 +81,7 @@ where
 {
     type InEvent = Void;
     type OutEvent = PeriodicIdHandlerEvent;
+    type Error = tokio_timer::Error;
     type Substream = TSubstream;
     type InboundProtocol = DeniedUpgrade;
     type OutboundProtocol = IdentifyProtocolConfig;
@@ -132,7 +133,7 @@ where
                 PeriodicIdHandlerEvent,
             >,
         >,
-        io::Error,
+        Self::Error,
     > {
         if let Some(pending_result) = self.pending_result.take() {
             return Ok(Async::Ready(Some(ProtocolsHandlerEvent::Custom(
@@ -146,15 +147,14 @@ where
         };
 
         // Poll the future that fires when we need to identify the node again.
-        match next_id.poll() {
-            Ok(Async::NotReady) => Ok(Async::NotReady),
-            Ok(Async::Ready(())) => {
+        match next_id.poll()? {
+            Async::NotReady => Ok(Async::NotReady),
+            Async::Ready(()) => {
                 next_id.reset(Instant::now() + DELAY_TO_NEXT_ID);
                 let upgrade = self.config.clone();
                 let ev = ProtocolsHandlerEvent::OutboundSubstreamRequest { upgrade, info: () };
                 Ok(Async::Ready(Some(ev)))
             }
-            Err(err) => Err(io::Error::new(io::ErrorKind::Other, err)),
         }
     }
 }
diff --git a/protocols/kad/src/handler.rs b/protocols/kad/src/handler.rs
index 8c1fadad..20c78a83 100644
--- a/protocols/kad/src/handler.rs
+++ b/protocols/kad/src/handler.rs
@@ -343,6 +343,7 @@ where
 {
     type InEvent = KademliaHandlerIn<TUserData>;
     type OutEvent = KademliaHandlerEvent<TUserData>;
+    type Error = io::Error; // TODO: better error type?
     type Substream = TSubstream;
     type InboundProtocol = upgrade::EitherUpgrade<KademliaProtocolConfig, upgrade::DeniedUpgrade>;
     type OutboundProtocol = KademliaProtocolConfig;
diff --git a/protocols/ping/src/dial_handler.rs b/protocols/ping/src/dial_handler.rs
index 5acfa1a5..abd7bfbe 100644
--- a/protocols/ping/src/dial_handler.rs
+++ b/protocols/ping/src/dial_handler.rs
@@ -33,7 +33,7 @@ use std::{
     time::{Duration, Instant},
 };
 use tokio_io::{AsyncRead, AsyncWrite};
-use tokio_timer::Delay;
+use tokio_timer::{self, Delay};
 use void::{Void, unreachable};
 
 /// Protocol handler that handles pinging the remote at a regular period.
@@ -153,6 +153,7 @@ where
 {
     type InEvent = Void;
     type OutEvent = OutEvent;
+    type Error = io::Error; // TODO: more precise error type
     type Substream = TSubstream;
     type InboundProtocol = DeniedUpgrade;
     type OutboundProtocol = Ping<Instant>;
diff --git a/protocols/ping/src/listen_handler.rs b/protocols/ping/src/listen_handler.rs
index 10a966ff..43c318a1 100644
--- a/protocols/ping/src/listen_handler.rs
+++ b/protocols/ping/src/listen_handler.rs
@@ -30,7 +30,6 @@ use libp2p_core::{
     upgrade::DeniedUpgrade
 };
 use log::warn;
-use std::io;
 use tokio_io::{AsyncRead, AsyncWrite};
 use void::{Void, unreachable};
 
@@ -72,6 +71,7 @@ where
 {
     type InEvent = Void;
     type OutEvent = Void;
+    type Error = Void;
     type Substream = TSubstream;
     type InboundProtocol = Ping<()>;
     type OutboundProtocol = DeniedUpgrade;
@@ -118,7 +118,7 @@ where
         &mut self,
     ) -> Poll<
         Option<ProtocolsHandlerEvent<Self::OutboundProtocol, Self::OutboundOpenInfo, Self::OutEvent>>,
-        io::Error,
+        Self::Error,
     > {
         // Removes each substream one by one, and pushes them back if they're not ready (which
         // should be the case 99% of the time).