The OAuth 2.0 bearer token specification, as defined in RFC 6750, allows any party in possession of a bearer token (a "bearer") to get access to the associated resources (without demonstrating possession of a cryptographic key). To prevent misuse, bearer tokens must to be protected from disclosure in transit and at rest. Some scenarios demand additional security protection whereby a client needs to demonstrate possession of cryptographic keying material when accessing a protected resource. This document motivates the development of the OAuth 2.0 proof-of-possession security mechanism.