The extent to which resource allocation policies are entrusted to user-level software determines in large part the degree of flexibility present in an operating system. In Hydra the determination to separate mechanism and policy is established as a basic design principle and is implemented by the construction of a kernel composed (almost) entirely of mechanisms. This paper presents three such mechanisms (scheduling, paging, protection) and examines how external policies which manipulate them may be constructed. |t is shown that the policy decisions which remain embedded in the kernel exist for the sole purpose of arbitrating conflicting requests for physical resources, and then only to the extent of guaranteeing fairness.