In this report the term ‘habitability’ is used to describe a condition that determines whether or not occupancy by operating or emergency response personnel of a ‘vital’ area of a nuclear installation during an accident is possible on a continuous or transient basis. Improvements and corrective measures in habitability can be designed into new installations or implemented at existing facilities. The report provides guidance on assessing habitability in existing installations and at the design stage for new plants and suggests actions to improve habitability in the event of an accident.