A sterilizable, stackable medical/dental instrument casette is formed of sterilization heat resistant plastic. A cover of the casette is closable, openable and separable from a tray of the casette. The tray is stackable on the cover, or other identical casette covers. Flexible, movable rails define compartments within the casette, and support and separate instruments in the casette. The cover is securely latchable to the tray.