Abstract: A digital tachometer which can operate to count the revolutions of any rotating device for which an electrical signal the frequency of which is related to the speed of rotation can be obtained is disclosed. The tachometer includes means for `normalizing` the input signal to provide a pulsed signal having a predetermined relationship with the speed of rotation, main counting means for counting the pulses, and means for displaying the accumulated count. The counters are controlled by logic circuits to count for a predetermined counting interval and the digital display is updated at the end of each counting interval. In addition to the main counting means there are provided interpolation means for generating a signal representing the number of revolutions performed by the device under test in the interval between each successive pulse of the "normalized" signal, to improve the accuracy of the tachometer.