This is a preview of ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, as it nears the end of the current revision process. Major additions to the previous Standard are: comfort zones for sedentary people wearing typical winter and summer clothing, information to design comfort zones for other clothing and activity levels, temperature drifts, and limits on the nonuniformity of thermal environmental parameters. The increased flexibility and information of the revision are conducive to the design and operation of comfortable and energy efficient buildings.