A method for diagnosing possible causes of changes in at least one control valve, in a plant of which the control valve is a part, or in a process which is carried out on the plant is proposed, wherein at least in a first and in a second time interval the position of the actuator is detected, an operating point or operating range is determined from the first and from the second time interval, these are compared, and possible causes of changes in the at least one control valve, in the plant or in the process are diagnosed from detected differences. Such a diagnosis does not require any additional sensors and makes it possible to detect emerging problems or undesirable developments solely from the operating state of the valve.