Particle Imaging Velocimetry (PIV) has been recognized as an essential and very useful technique for analyzing two- or three- dimensional complex flow fields and two-phase fluid flows. PIV techniques promise to give improved results because they provide a visual solution taking the total field into account. In addition, PIV offers many advantages for the study of fluid flow. PIV is a measuring technique that allows us to capture the flow velocity of whole flow fields in a fraction of a second. Hence, PIV has become more and more popular and it has rapidly spread in the world being recognized as the most advanced flow velocimetry because of its strong merits. Its application ranges have been expanding to measure turbulent flow, multiphase flow, internal flow of fluid machines, bioengineering, medical engineering, environmental engineering, energy engineering, development of new materials, sports science, life science, mechatronics, robotics. Thus, PIV is a very promising powerful tool in the study of the structure of flows. In this book, the principles and the typically used methods of PIV measurement are summarized. The classification of PIV methods is also explained.