—ADP plays a key role in hemostasis and thrombosis. Despite its early identification in 1961 as the first known aggregating agent, the molecular basis of ADP-induced platelet activation is only beginning to be understood. The present review proposes a model of 3 purinergic receptors contributing separately to the complex process of ADP-induced platelet aggregation: the P2X₁ ionotropic receptor, responsible for rapid influx of ionized calcium into the cytosol; the P2Y₁ metabotropic receptor, responsible for mobilization of ionized calcium from internal stores, which initiates aggregation; and an as-yet-unidentified P2Y receptor coupled to G_αi2, which is essential for the full aggregation response to ADP. It is probable that this as-yet-unidentified receptor is the molecular target of the ADP-selective antiaggregating drugs ticlopidine and clopidogrel. In addition, it is probably defective in patients with a bleeding diathesis that is characterized by selective impairment of platelet responses to ADP.