The flow rate through a channel or an orifice increases with the increase of the pressure difference ∆p between the two sections, one upstream and the other downstream. This velocity is called "sonic velocity"; in critical flows, the fluid reaches, at the outlet section, a velocity that equals its sonic velocity, which is a function of the critical pressure and of other physical conditions at the outlet section. In this sense, the determination of the critical flow conditions is relatively easy in single-phase flows, whereas in multiphase flows, it becomes more difficult owing to the interactions among the phases, which play a determinant role. The separate-phase models, adopting a much more detailed physical description, have a much wider validity. The differences among the proposed models are connected with the schematizations of thermal and mechanical interactions between the phases and with the definition of the critical condition.