In this paper, given a consistent description of the mathematical theory of the interfacial layer, which includes a formal mathematical formulation of the problem; formulation of constitutive equations (physical equations); solution to the problem of identification of parameters that define the nonclassical effects; Analytical evaluation of the properties of the interfacial layer, assess the impact properties of the interfacial layer on the effective characteristics of composite qualitative analysis theory. The presentation is based on the analytical review conducted by the studies that using a variational formulation developed correct and consistent model options, taking into account the scale effects are presented models of cohesive and adhesive interactions in the environments are examples of modeling non-classical effects in fracture mechanics, mechanics of thin-film structures, mechanics disperse composites.