Fundamental studies of friction between solid surfaces are bedevilled by their inevitable roughness. However recent developments in nanotribology through molecular dynamics, the surface force apparatus, and the atomic force microscope have made possible direct examination of adhesion and friction at a single asperity in which the real and apparent contact areas coincide. In this paper the continuum theories of straight adhesion ata spherically tipped asperity are reviewed and a "map" presented which indicates the conditions under which the different theories apply. The interaction between friction and adhesion is then considered from the point of view of "fracture mechanics". This approach has the advantage of clearly separating the roles of continuum mechanics and physics. The former is used to calculate the "elastic energy release rate" G as the adhesion is broken; the work of adhesion G(c) can only be found from physical considerations.