A semi-theoretical model of minimum wetting rate for gravity-driven liquid films under isothermal conditions is presented. It is based on the experimental observations that disturbed liquid layers break up more easily than smooth ones and that the contact angle is a variable quantity. The theory comprises the main features of the three classical models used so far : flow distortion, force balance considerations, minimum-energy requirement. Some experimental evidence supports the main ideas of the model. There is a reasonably good agreement with respect to the boundaries of the transition region.