T he critical role for ADP in arterial thrombogenesis was established by the clinical success of P2Y₁₂ antagonists, currently used at doses that block 40–50% of the P2Y₁₂ on platelets. This study was designed to determine the role of P2Y₁₂ in platelet thrombosis and how its complete absence affects the thrombotic process. P2Y₁₂-null mice were generated by a gene-targeting strategy. Using an in vivo mesenteric artery injury model and real-time continuous analysis of the thrombotic process, we observed that the time for appearance of first thrombus was delayed and that only small, unstable thrombi formed in P2Y₁₂^–/– mice without reaching occlusive size, in the absence of aspirin. Platelet adhesion to vWF was impaired in P2Y₁₂^–/– platelets. While adhesion to fibrinogen and collagen appeared normal, the platelets in thrombi from P2Y₁₂^–/– mice on collagen were less dense and less activated than their WT counterparts. P2Y₁₂^–/– platelet activation was also reduced in response to ADP or a PAR-4–activating peptide. Thus, P2Y₁₂ is involved in several key steps of thrombosis: platelet adhesion/activation, thrombus growth, and stability. The data suggest that more aggressive strategies of P2Y₁₂ antagonism will be antithrombotic without the requirement of aspirin cotherapy and may provide benefits even to the aspirin-nonresponder population.