A cassette for steam sterilizing of instruments comprises a lid and a tray, the tray having an inlet and an outlet for the cassette for communication with a sterilization apparatus, and the lid having a seal configured to interface with the tray to maintain either pressure or vacuum conditions for an interior of the cassette. The seal is configured to allow the lid to move between a vacuum position and a pressurized position without exposing the cassette interior to an outside environment. The inlet and outlets have two sets of valves and seals that ensure that the cassette interior remains sterile after the sterilization process is complete and during cassette storage. One set of valves also allows access to the cassette interior for the steam sterilization cycle.