The development of a surface shear layer, following the sudden application of wind stress, is governed by a steady increase in total momentum and a downward penetration of the shear flow region. Analogy with the turbulent boundary layer suggests a two-layer conceptual model with an interface layer at the surface and an outer layer below. The growth of the outer layer may be supposed to be described by an entrainment law and an eddy viscosity proportional to local length and velocity scales. Matching to an interface layer yields a surface velocity increasing only very slowly with time and having a typical magnitude of 30u*. A comparison with constant viscosity and space dependent viscosity models shows that a unique feature of the developing shear layer model is the vertical penetration of a shear impulse, followed by low shear in the developed region of the flow. Some scant experimental data seem to support this model, but systematic experimental work on the problem is yet to be carried out.