Two-phase releases can occur through many different types of breaches, such as wall cracks, orifices, short or long pipes, or piping systems. They include subcooled liquids which flash during their expansion, saturated vapours or two-component flows. No general correlation or model is available to predict choked two-phase flows in such a wide variety of situations. On the contrary, many different models or correlations exist, each having been validated on a particular set of data. In an attempt to help the user find his way through this complex subject, this paper presents the rationale behind the development of models. Several particular critical flow models are described, in the development of which the author has been involved during the past 20 years, and the basic assumptions and equations as well as the validation studies are reported.