In fluid mechanics, multiphase flow is the simultaneous flow of materials with two or more thermodynamic phases. Virtually all processing technologies from cavitating pumps and turbines to paper-making and the construction of plastics involve some form of multiphase flow. It is also prevalent in many natural phenomena.

These phases may consist of one chemical component (e.g. flow of water and water vapour), or several different chemical components (e.g. flow of oil and water). A phase is classified as continuous if it occupies a continually connected region of space (as opposed to disperse if the phase occupies disconnected regions of space). The continuous phase may be either gaseous or a liquid. The disperse phase can consist of a solid, liquid or gas.

Two general topologies can be identified: disperse flows and separated flows. The former consists of finite particles, drops or bubbles distributed within a continuous phase, whereas the latter consists of two or more continuous streams of fluids separated by interfaces.