This document describes the potential benefits when applications enable Explicit Congestion Notification (ECN). It outlines the principal gains in terms of increased throughput, reduced delay and other benefits when ECN is used over network paths that include equipment that supports ECN-marking. It also identifies some potential problems that might occur when ECN is used. The document does not propose new algorithms that may be able to use ECN or describe the details of implementation of ECN in endpoint devices, routers and other network devices.