[en] The practical problem associated with the critical flow phenomenon is the occurrence of a maximum flowrate for given upstream conditions. An engineering definition of criticality is proposed and its consequences discussed. The paper presents the theory of critical flow, within the framework of the current, one-dimensional, quasilinear, first order partial-differential equation models. The difficulties raised by most current models, which are incomplete two-fluid models (i.e. two-fluid models in which some equations are replaced by assumptions on the evolution of the flow variables) are brought out. The importance of nonequilibrium effects and of the mathematical form of the transfer laws is stressed. The possibility of a pseudo-criticality is discussed in relations with wave damping. Finally, since numerical calculations are made with discretized equations, the transcription, in a discretized set of the criticality conditions, is examined. It is shown that criticality may be hidden in the discretized set and that the criticality conditions must be dealt with expressly. 34 references