This chapter outlines the hemostatic response to vascular damage in the carotid artery and considers the endogenous hemostatic factors that may determine the likelihood of embolization in patients. It discusses the mechanisms involved in thrombus formation and stabilization. Platelets provide a reinforced loop in the generation of a thrombus, providing a source of thrombin to recruit new platelets and propagate clot formation. The mechanism of stabilization of a thrombus by P-selectin appears to be partly stabilization of platelet-platelet aggregates but mostly through recruitment of leucocytes via interaction of P-selectin with PSGL-1. Many factors are involved in forming a stable thrombus and consequently there are many candidates for regulating the risk of embolization. Antiplatelet and antithrombotic therapies are of benefit in limiting the growth of thrombus within the carotid vessel. In particular, adenosine diphosphate (ADP) seems to have a very specific role in regulating embolization.