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The basic principles of the laser Doppler velocimeter, a new instrument for measurement of localized flow velocities in gases and liquids, are discussed in detail. Velocity measurements are made by detecting the Doppler shift in monochromatic laser light which is scattered from small contaminant particles in the fluid. A gas laser serves as the light source, and the Doppler shift is detected by optical heterodyne techniques. The latest fluid flow measurements obtained with the laser Doppler velocimeter are presented.