A description is given of an interferometer capable of non-contact measurements of the displacement and velocity of solid surfaces. The interferometer may be used with any C.W. laser in the visible region and no treatment of the surface is usually necessary. To over-come the problem of directional ambiguity, the reference beam is frequency shifted electro-optically using a Kerr cell device. The output is a frequency modulated signal that may be processed to obtain instantaneous velocity by a ratemeter or more sophisticated frequency recording device. Displacement measurements may be made by counting cycles and continuously subtracting counts derived from the shift frequency. For instantaneous movement a precision of one count or a quarter of a wavelength is obtained but mean displacements may be registered with greater accuracy. In another mode of operation, the output of the interferometer is fed back to control the frequency shift to "phase-lock" the interferometer. This enables very small movements at high frequencies to be recorded in the presence of the much larger low frequency vibrations normally encountered in the laboratory. This technique has been successfully used in the calibration of ultrasonic transducers.