Empirical mass transfer formulas for evaporation from lakes and from pans at ground level are compared with the theoretical solution based on semiempirical turbulent diffusion theory in conjunction with Reynolds' analogy and the power law for the wind profile. It was found that although the theory has its limitations, it is a useful tool for the study of evaporation, especially under conditions of lateral advection. The empirical formulas should be adequate for many design problems.