The integrins are a family of alpha,beta heterodimeric receptors that mediate dynamic linkages between extracellular adhesion molecules and the intracellular actin cytoskeleton. Integrins are expressed by all multicellular animals, but their diversity varies widely among species; for example, in mammals, 19 alpha and 8 beta subunit genes encode polypeptides that combine to form 25 different receptors, whereas the Drosophila and Caenorhabditis genomes encode only five and two integrin alpha subunits respectively. Thousands of studies over the last two decades have investigated the molecular, cellular and organismal basis of integrin function. Gene deletion has demonstrated essential roles for almost all integrins, with the defects suggesting widespread contributions to both the maintenance of tissue integrity and the promotion of cellular migration. Integrin-ligand interactions are now considered to provide physical support for cells in order to maintain cohesion, to permit the generation of traction forces to enable movement, and to organize signalling complexes to modulate differentiation and cell fate. Animal-model studies have also shown that integrins contribute to the progression of many common diseases, and have implicated them as potential therapeutic targets. The use of anti-integrin monoclonal antibodies and ligand-mimetic peptides has validated this suggestion for inflammatory, neoplastic, traumatic and infectious conditions. Thus, to understand more about the mechanisms underlying tissue organization and cellular trafficking, and to identify approaches for regulating these processes in disease, there is intense interest in determining the molecular basis of integrin function. It is important to state at the outset that the tertiary structure of the integrin dimer is unknown. Our current understanding of the molecular basis of integrin function is therefore compiled from the results of a large number of studies that have employed a wide range of complementary technologies.