A computer protection mechanism is a set of tools for controlling the actions of computations and safeguarding stored information. This paper describes a new mechanism, COPS, which is a kernel of data structures, primitive operations, and a monitor and is used to specify and enforce the capabilities of actors (processes and procedures). COPS can be used to implement a variety of security policies and systems and to enhance software reliability. Its tools are sufficient to solve problems in the areas of isolation, controlled sharing, restricted access, mutually suspicious interaction, and confinement. Key Words and phrases: protection, kernel, operating system, security, access control.