Chapter 1 provides an overview of the concepts and exemplified applications of multiphase flows. It illustrates the distinctly different transport patterns or phenomena of individual phase in a multiphase flow, which have either naturally caused or intentionally designed consequences.

The chapter conveys the basic definitions of a multiphase flow, the phase interactions, and the associated modeling approaches, which include the difference between a multiphase flow and a multicomponent single-phase flow, the difference between a dilute-phase multiphase flow and a dense-phase multiphase flow, the difference between a continuum phase and a discrete phase in describing the flow regimes, and the difference in Eulerian–Lagrangian modeling and Eulerian–Eulerian modeling. Some interesting and unique phenomena of multiphase flows are discussed by case studies.