Solid, liquid, and vapour are the three natural phases of materials. A multiphase system contains materials of two or more phases. Since the early 1980s, the design of more advanced instruments and the application of more scientific measurement techniques have led to measurement and quantification of fundamental parameters that improved our understanding of multiphase systems and processes. For the size measurements of particles, one chooses a measuring technique, such as photography or sieve measurements, and also chooses size intervals which are not necessarily of equal magnitude. Most of the practical applications of dispersed multiphase flows involve time-dependent flows. When the characteristic time of the flow transients is of the same order of magnitude as the response time of bubbles, drops, or particles, a transient equation should be used for the exchange of momentum between the carrier fluid and the dispersed phase. The chapter also presents an overview of the key concepts discussed in this book.