An instrument is described for measuring both particle and droplet size distributions. Based on Fraunhofer diffraction of laser light, the measurement is fast, reliable, and easy to make. Hardcopy results include a characteristic diameter, distribution width, and a 15-segment histogram of the size distribution. Following a brief review of sizing techniques and the importance of measuring size distributions, a technical discussion is presented on the principles of the instrument. Configuration of the instrument is discussed. Five examples of commercial application are presented covering spray and particle samples.