Abstract: An environmental control system for improving the energy efficiency of structures is disclosed. A structure having a heating and/or cooling plant, of any conventional type, is improved by providing, in each subdivision of the structure, such as a room, a temperature sensor, and an occupancy sensor. The outputs of the temperature and occupancy sensors are received by a control subsystem whose output is a composite function of the output signals of the occupancy and temperature sensors and which controls the heating and/or cooling system of the structure to maintain each room at a first temperature when the room is occupied, and at a second temperature when the room is unoccupied.