The Containment Systems Experiment (CSE) program is reviewed, with emphasis on the inherent processes that remove fission products from containment atmospheres and reduce their leakage to the environment. The CSE containment vessel was sized to represent a one-fifth linear scale model of a typical 1000-MW(electric) pressurized water reactor. Nineteen tests were performed in a steam-air atmosphere simulating conditions after a loss-of-coolant accident.