Noninterference was introduced by Goguen and Meseguer to provide a foundation for the specification and analysis of security policies. The intuitive notion that a security domain U is noninterfering with a security domain V is conveyed by stating that no action performed by U can influence subsequent outputs seen by V. This lecture traces the history of this intriguing concept and shows how to reduce the satisfiability of noninterference to the solution of a finite set of equations. This is accomplished by examining properties of special sets called basis elements which are generated from the interference relationship defined from the allowable flows of information and applying the principles of finite state machines.