It is now well established that laser flow-measuring systems have important advantages over more conventional techniques both for industrial and laboratory applications. These fundamental advantages are indicated by the enormous research effort which has gone into their development over the last decade and by the number of commercial systems which have become available. Although the field is still developing, the most important theoretical results required for relating the system outputs to the fluid flow parameters have now been formulated and a book on the subject therefore seems timely. In the text we have tried to collect together the most important results both from our own papers and from publications by other authors and to present these in a concise and easily readable form. Emphasis has been placed on the fundamental theory and limitations associated with the various techniques rather than on detailed description of specific systems. We have also included a number of new results on areas such as photon counting in turbulent and periodic flows, frequency domain and time domain analysis of laser Doppler velocimeter signals, effect of background noise on system performance, and on cross-correlation techniques for diffusing flows.