Summary. Integrins are a ubiquitous family of non-covalently associated α/β transmembrane heterodimers linking extracellular ligands to intracellular signaling pathways  [Cell, 2002; 110: 673]. Platelets contain five integrins, three β1 integrins that mediate platelet adhesion to the matrix proteins collagen, fibronectin and laminin, and the β3 integrins αvβ3 and αIIbβ3  [J Clin Invest, 2005; 115: 3363]. While there are only several hundred αvβ3 molecules per platelet, αvβ3 mediates platelet adhesion to osteopontin and vitronectin in vitro  [J Biol Chem, 1997; 272: 8137]; whether this occurs in vivo remains unknown. By contrast, the 80 000 αIIbβ3 molecules on agonist-stimulated platelets bind fibrinogen, von Willebrand factor, and fibronectin, mediating platelet aggregation when the bound proteins crosslink adjacent platelets  [J Clin Invest, 2005; 115: 3363]. Although platelet integrins are poised to shift from resting to active conformations, tight regulation of their activity is essential to prevent the formation of intravascular thrombi. This review focuses on the structure and function of the intensively studied β3 integrins, in particular αIIbβ3, but reference will be made to other integrins where relevant.