The condition of internal mechanical equilibrium of a curved surface layer is derived, and its application to practically important cases of incomplete formation of the surface layer (as is sometimes the case, for example, in thin films) is considered. The notion of a local disjoining pressure is introduced, and the equilibrium condition for a variable-thickness thin film is obtained; this condition is valid both in the absence and in the presence of external fields. The cases of a wedge-shaped film, cylindrical film, spherical film, and transition zone of a wetting film are analyzed.