A theoretical analysis for a laser Doppler anemometer is presented. Starting with Mie scattering theory, expressions are derived for the photocurrent and spectrum due to a single moving scattering particle. From these expressions relations defining the region from which signals are received, the spectrum bandwidth and the spectrum amplitude are obtained. The results are generalized to the signal due to an arbitrary number of randomly positioned scattering particles and treated statistically to examine the effects of particle concentration on the mean square spectrum amplitude and signal-to-noise ratio. Noise in this context refers to spectrum fluctuations arising from the randomness of scatterer positions and concentration fluctuations.