Thrombosis reflects an imbalance between procoagulant and anticoagulant mechanisms. In some cases, thrombotic lesions are precipitated by gross changes in blood flow, vascular wall integrity, or systemic levels of coagulation factors. In other cases, thrombosis is induced by functional changes within the endothelium. Endothelial cells express a wide variety of factors that contribute to hemostasis, including procoagulants, anticoagulants, cell adhesion molecules, vasomotor substances, and cell survival signals. Because the endothelium displays a remarkable diversity of structure and function, the relative contribution of any one of these factors to the hemostatic balance varies between different vascular beds. In this review, we emphasize the heterogeneous nature of endothelial cell function. We then examine the role of endothelial diversity in modulating the phenotypic expression of thrombotic disorders.