The ability to routinely make simultaneous measurements of the size and velocity of individual spherical particles is identified as a crucial component of advances in the science and technology of multiphase systems and processes. Since it is the dual beam interferometer identified here as the phase-Doppler particle-sizing velocimeter (PD) that has provided this capability, and since 1995 marks twenty years from a seminal paper by Durst and Zare, a historical review is warranted and is presented. The roots and context of the development of PD are traced from the days of laser Doppler velocimetry (LDV) in the early 1970＇s through work culminating at several laboratories in the mid 1980＇s. A number of important contributions and the engineers and scientists responsible for those contributions are recognized.