This paper addresses the origin, structure and properties of the interphase in adhesion. By interphase is meant that interfacial region between bulk adhesive and bulk adherend over which the local density displays a spatial gradient. In such a region all the local thermodynamic properties, including the mechanical properties, will be a function of distance from the surface. The question of how large these variations may be both in terms of magnitude and range is clearly a matter of some import in adhesive technology. The term ＇＇local＇＇ is a key consideration because meaningful discussion of the interphase can hardly be contemplated without explicit recognition that matter in the interphase should be regarded properly as existing in a thermodynamically metastable small system of low dimensionality. The results of a number of our numerical and theoretical studies, namely molecular dynamics simulation of realistic interfaces and nonlinear dynamical analysis, have been used to investigate the nature of materials close to surfaces. It has been found that the form and range of the density profile are indeed sensitive to the chemical nature of the surface but it has not been possible to rationalize the existence of the macroscopic long-range interphase on the basis of the usual type of spatial correlation. Recognizing that many adhesive bonds are formed using a reactive resin system we have investigated the effects of nonlinearities in the reaction kinetics on the structure and morphology of the interphase. When these factors are taken into account, a theoretical description of the interphase is possible in which the coupling between the various distance scales arises in a very natural fashion.