A thermostat (100; 208; 602), comprising: a housing; a processing system disposed within the housing and coupled to a user interface, the processing system being configured to be in operative communication with one or more temperature sensors for determining an ambient air temperature, in operative communication with one or more input devices including said user interface for determining a setpoint temperature value, and in still further operative communication with a heating, ventilation, and air conditioning (HVAC) system to control the HVAC system based at least in part on a comparison of a measured ambient temperature and the setpoint temperature value; and at least one occupancy sensor in operative communication with the processing system, the thermostat including an away-state feature in which the thermostat enters into an away-state mode of operation upon a determination by the processing system based on readings acquired by the at least one occupancy sensor that an away-state criterion indicative of a non- occupancy condition for an enclosure in which the thermostat has been installed has been satisfied, the away-state mode of operation including an automated setpoint temperature setback mode; wherein the processing system is further configured to automatically determine, without requiring user input, whether to activate said away-state feature for the enclosure in which the thermostat has been installed, and characterised by said activation determination comprising: receiving readings from the at least one occupancy sensor during a trial period; comparing information derived from said trial period readings to a threshold criterion to establish whether sufficiently true indications of occupancy conditions were sensed by the at least one occupancy sensor during the trial period; and enabling said away-state feature of the thermostat only if it is determined that sufficiently true indications of occupancy conditions were sensed during the trial period. a housing; a processing system disposed within the housing and coupled to a user interface, the processing system being configured to be in operative communication with one or more temperature sensors for determining an ambient air temperature, in operative communication with one or more input devices including said user interface for determining a setpoint temperature value, and in still further operative communication with a heating, ventilation, and air conditioning (HVAC) system to control the HVAC system based at least in part on a comparison of a measured ambient temperature and the setpoint temperature value; and at least one occupancy sensor in operative communication with the processing system, the thermostat including an away-state feature in which the thermostat enters into an away-state mode of operation upon a determination by the processing system based on readings acquired by the at least one occupancy sensor that an away-state criterion indicative of a non- occupancy condition for an enclosure in which the thermostat has been installed has been satisfied, the away-state mode of operation including an automated setpoint temperature setback mode; wherein the processing system is further configured to automatically determine, without requiring user input, whether to activate said away-state feature for the enclosure in which the thermostat has been installed, and receiving readings from the at least one occupancy sensor during a trial period; comparing information derived from said trial period readings to a threshold criterion to establish whether sufficiently true indications of occupancy conditions were sensed by the at least one occupancy sensor during the trial period; and enabling said away-state feature of the thermostat only if it is determined that sufficiently true indications of occupancy conditions were sensed during the trial period.