A phenomenological theory of wetting layer systems based on the van der Waals theory of interfaces is developed. For classical critical exponents the local composition profile and the wetting layer thickness are determined analytically and their behavior near a critical point is examined. The model is then extended to the case of nonclassical critical exponents, where numerical solutions for the composition profile and the wetting layer thickness are determined. The nonclassical model of the wetting layer is then applied to a well-studied experimental system and the results for the coefficient of ellipticity are compared with experimental measurements.