The present invention relates to a method and an apparatus for controlling the temperature of heating and cooling plants having a heat or cooling source of adjustable rated temperature via particularly a microprocessor-controlled control system. The actual outside temperature determined by an outside temperature sensor is not used as reference temperature but instead there is used a theoretical mean outside temperature stored at specific scanning times of a calendar year. Said theoretical mean outside temperature is stored in the form of a characteristic diagram in a non-erasable memory for one or several climate zones. By correction of the stored values of the theoretical mean outside temperature particularly on the basis of the thermal load of the system determined via the rate of heating or cooling of the heat or cooling source as well as perhaps the correction of the daily course of the outside temperature on the basis of likewise stored values the rated temperature of the heat or cooling source can be so controlled that it corresponds to the actual outside temperature conditions without requiring an outside temperature sensor. With regard to its construction, installation and putting it into operation as well as from the economic point of view the apparatus is particularly advantageous.