A concise historical account of the development of adhesion theories and a critical discussion of their contemporary relevance are given. The pioneering work of McBain and Hopkins in 1925 led to the development of the modern adsorption and mechanical theories of adhesion. Somewhat later, there were important contributions from Russia where workers introduced the electrostatic theory (Deryaguin) and the diffusion theory of adhesion (Voyutskii). Recent developments in contact mechanics, molecular dynamics, and, in particular, surface analysis have provided considerable insight into the nature of the interface and interfacial region in adhesive joints. These suggest that adsorption, mechanical, and even diffusion effects cannot be completely isolated from one another. It is argued that each theory is best regarded as emphasising a different aspect of a more comprehensive model which, in principle, relates molecular dispositions in the region of the interface to macroscopic properties of an adhesive joint.