—Authentication establishes the identity of one party to another. Most commonly authentication establishes the identity of a user to some part of the system, typically by means of a password. More generally, authentication can be computer-to-computer or process-toprocess and mutual in both directions. —Access control determines what one party will allow another to do with respect to resources and objects mediated by the former. Access control usually requires authentication as a prerequisite. —The audit process gathers data about activity in the system and analyzes it to discover security violations or diagnose their cause. Analysis can occur offline after the fact or online in real time. In the latter case, the process is usually called intrusion detection.