Abstract: A method and apparatus for controlling the operation of a well involves locating a pressure monitoring device downhole and arranging it to be operative: to monitor a characteristic pressure profile of a well during a certain time span controlled by a downhole clock associated with the device; to monitor the temperature downhole in the region of the clock; to calibrate a time reading of the downhole clock with reference to the monitored temperature to correct for drift between the clock time and real time resulting from the downhole elevated temperature; and to respond and generate a triggering output when a significant deviation to the pressure profile is introduced into the well as a control signal to the monitoring device, by comparing two separate pressure profiles within the time span, the device only generating the triggering output when the same pressure profile is monitored twice within the time span.