Laser Doppler velocimetry (LDV) is a well-established technique for making non-intrusive measurements of particle velocities in a wide variety of situations. However, the particle velocities so measured are often much more useful if the sizes of the individual particles measured are also known. While it is possible to analyse standard LDV signals to obtain measurements of particle size simultaneously with those of particle velocity in some circumstances, this method is difficult to use on a routine basis. The recent extension of LDV usually known as the phase-Doppler technique has, however, made simultaneous size and velocity measurements much more straightforward. The principles of this method and its strengths and limitations will be discussed. In particular, although in principle the method is restricted to cases where the particles or droplets are nearly spherical, evidence will be presented to show that this restriction can be relaxed if the data is processed suitably. It will be shown that as a by-product of this, it is also possible to extract information about the degree of sphericity of the material studied. Finally, application of this technique to a number of industrially important flows will be considered, and some of the problems encountered will be described.