Instruments based on ultrasonic spectrometry are becoming increasingly popular for determining the droplet size distribution of emulsions. They have major advantages over alternative technologies because they are capable of analyzing emulsions which are concentrated and optically opaque, without disturbing the sample. This article reviews the physical principles of ultrasonic particle sizing, the experimental methods commonly used to make ultrasonic measurements, and the mathematical equations used to convert ultrasonic measurements to droplet size distributions. Ultrasonic spectrometry is compared to other particle-sizing techniques so that colloid scientists can decide whether it is suitable for their particular application.