We examine entrainment across stratified interfaces as a fluid mechanical process which depends on the local Reynolds, Schmidt, and Richardson numbers. We show that the observed dependence of entrainment velocity on the local Richardson number over a wide range of fluid dynamical parameters can be explained in terms of simple models of the turbulence at the interface. In the particular case of entrainment into nocturnal stratocumulus we derive an analytic, linear model of the radiatively driven convective turbulence in the layer. The model is applied to the observations of Caughey and Kitchen (1984), and it is shown that this model reproduces the gross features of the convection and the magnitude of the entrainment rate.