Experience has established a major role for platelets in the pathogenesis of a variety of thromboembolic disorders. Despite advances in several areas, many problems remain. The relevance of platelet function testing to thromboembolic disorders needs clarification. Whether the association of enhanced platelet function and the aforementioned disorders represents cause, effect or nonspecific accompaniment is unknown. The concept of identifying individuals at risk by platelet function testing is attractive but unproved. Whether such individuals would benefit from prophylactic antiplatelet therapy is also unknown. For treatment of most established thromboembolic disorders as well as prophylaxis, the place of antiplatelet drugs is not established. Whether this form of therapy is superior to conventional treatment, adjunctive or of no benefit is not resolved in most instances. Also, the most appropriate antiplatelet drug or combination of drugs, and in which dosages for specific disorders, remains unclear. Well designed, prospective clinical investigation, although cumbersome and time-consuming, will be necessary to answer most of these questions. A final problem area concerns research into the pharmacology of platelet inhibition. It is probable that the ideal antiplatelet agent remains undiscovered. Currently, several investigators are looking into the possibility of manipulating prostaglandin metabolic pathways in hopes of specifically blocking thromboxane generation while allowing production of metabolites inhibitory to aggregation. This exciting approach and other investigations into the biochemical basis of platelet function should lead to the discovery of new antiplatelet agents.