Theories of adhesion are based on adsorption and wetting, on diffusion, on donor/acceptor or electrostatic interactions and on simple mechanical interlocking of the adhesive into irregularities of its substrate. The principal contributions of recent work to these theories are outlined. Joint strength is also a matter of stress distribution and knowledge of this has been advanced both by formal mathematical methods and by the use of finite element analysis. The tack of adhesives is now better understood as involving the interaction of both bulk and surface properties with the rate of separation. Fracture mechanics have been applied to the investigation of the failure of structural adhesives in humid environments. The properties determining adhesive behavior comprise Tg, solubility parameter, surface free energy, viscosity and microstructure of the polymer. The latter, more recently explored, is examined in detail. The structure of adhesives for use at high temperature and changes in technology demanded by impending legislation to reduce solvent conclude the review. -- AATA