This chapter is devoted to the methodology of particle image velocimetry (PIV) techniques and its applications to multiphase flow systems. It reviews, first, the fundamental issues of a conventional PIV with considerations of improvements of spatial resolution and accuracy; second, the state of the art in various types of PIV techniques from the viewpoint of dimensions and velocity components of the measurement, the flow passage scales, and the hardware components of the system; third, the state of the art in some issues about the measurement of multiphase flow systems using PIV techniques. The multiphase flows to which the applications of PIV techniques are discussed include liquid-liquid two fluid flows, gas-liquid two-phase flows, and particle-laden multiphase flow systems. The emphasis in this chapter is on the fruitful methodology of PIV techniques that emerge in the recent publications instead of the detailed discussions on any individual research topic of the measurement target of PIV. The purpose is to provide an overall instructive introduction and guidance to the PIV techniques and its applications particularly in the research field of multiphase flows. To this end, fruitful examples of PIV measurements of free-surface liquid flows, bubbly flows, particle-laden multiphase flows, etc., are elucidated.