Ticlopidine and clopidogrel are orally active antiplatelet agents that block ADP-mediated platelet activation. The proaggregatory effects of other platelet agonists are also attenuated by administration of ticlopidine or clopidogrel; however, this generalized inhibition of platelet activation may arise from interference with the synergy between platelet-derived ADP and other proaggregatory agents. Ticlopidine and clopidogrel do not display antiplatelet activity when added to blood or purified platelets; hence, biotransformation of these agents appears to be a prerequisite for inhibition of platelet activation. The purported active derivatives have not been identified. A variety of preclinical and clinical investigations have demonstrated that ticlopidine and clopidogrel are useful for the treatment and/or prevention of platelet-dependent thrombotic disorders.