An overview is presented of optical techniques used to detect ultrasound propagating in solids. Typically, the sensor system may be based on a beam deflection technique, a Michelson interferometer or a Fabry-Perot interferometer. In some cases, the systems offer a flat wideband frequency response up to frequencies in excess of 100 MHz. The techniques used are non-contacting and possess none of the ultrasonic ringing characteristics associated with more conventional ultrasonic sensors. Estimates of some minimum displacement sensitivities are presented