A qualitative and quantitative theory of the MOS transistor in saturation is developed, taking into account the fact that the carrier concentration in the drain region is not negligible. With reference to the behavior in saturation, an injection level is defined. This level is directly related to two parameters: the drain saturation field EDSand the effective depth of the drain region xD. A division of the current domain in low, medium, and high levels is proposed. For low injection levels (for which the saturation field is smaller than the critical field), an iterative procedure for the calculation of the drain saturation conductance is given. A method for determining the channel configuration is presented. Inconsistencies in the pinchoff concept are revealed by the calculation of this configuration and by the analysis of the validity domain of the equations based on gradual approximation.