The authors discuss supervisory control of probabilistic discrete-event systems in this paper. Supervisors for such systems will take probabilities of event occurrences into consideration in making control decisions. The authors introduce a new concept called "epsilon-containment of a given language" and subsequently study its algebraic properties. They discuss standard supervisor (with only enablement/disablement) synthesis problems and reset supervisor (with recovery mechanism) synthesis problems for both fully observed and partially observed systems.