Abstract: A water level controller for a boiler has electrical probe means (capacitive or conductive) to be arranged in the boiler to detect electrically the water level therein. An oscillator circuit generates an oscillator signal which first and second circuit means receive. When a capacitance probe is used, the first circuit means has a variable gain amplifier in which the gain is determined by variations in the capacitance of the probe according to the water level. The second circuit means comprises a variable gain amplifier in which the gain is determined by a variable resistance the value of which is representative of the desired operating water level. Third circuit means obtains a difference signal resultant from the output signals of said first and second circuit means, which is representative of the difference between the actual water level in the boiler and the desired operating level of the water in the boiler. Relays open and close the water supply respectively.