The Scripps Research lnstitute, La Jolla, CA, USA lntroduction interact with one another by appropriate stimuli. However, thrombi more likely develop in vivo by progressive accrual of platelets transferring individually from the" mobile" phase of circulating blood to the" immobile'vessel wall-in other words, as a result of continued adhesion on a dynamic substrate rather than as cohesion of multiple platelets in zuspension. Platelet responses to vascular injury depend on the binding of membrane receptors to immobilizrn or soluble ligands and are modulated by stimulus-coupled biochemical and cytoskeletal responses, representing the paradigm of adhesion for circulating vascular cells. Under the effects of flow, the latter can interact efficiently with the vessel wall only by withstanding opposing drag forces.