An open operating system, which provides a high degree of programming flexibility and efficiency, generally requires that all programs be written in a single language and provides no protection other than that which is available from the compiler. It is noted that these limitations become unacceptable on a workstation that must run untrusted software written in many different languages. Psyche, an open operating system designed to make the most effective possible use of shared-memory multiprocessors and uniprocessor machines, is presented. It combines the flexibility of an open operating system with the ability to write in multiple languages and to establish solid protection boundaries. It also provides the efficiency of an open operating system for programs that do not require protection.