The main control room at a nuclear power station must remain habitable during a variety of plant conditions and postulated events. The control room habitability requirement and the function of the heating, ventilating, air-conditioning, and air treatment system are to control environmental factors, such as temperature, pressure, humidity, radiation, and toxic gas. Habitability requirements provide for the safety of personnel and enable operation of equipment required to function in the main control room. Habitability as an issue has been gaining prominence with the Advisory Committee on Reactor Safeguards and the Nuclear Regulatory Commission since the incident at Three Mile Island. Their concern is the ability of the presently installed habitability systems to control the main control room environment after an accident. This paper discuso ses main control room HVAC systems; the concerns, requirements, and results of NRC surveys and notices; and an approach to control room habitability reviews.