The concept that leukocytes play an active role in hemostasis and thrombosis has only recently been accepted. Leukocytes may influence coagulation directly, by the production of procoagulant and anticoagulant molecules, or indirectly, by actions on vascular cells including platelets, endothelial cells, and other leukocytes. This review examines the role of leukocytes in coagulation with an emphasis on regulation of leukocyte function by interactions, with platelets. Activated platelets may serve both to localize leukocytes in areas of thrombosis and to modulate their function. Over the past year, several in vitro studies further defined molecular mechanisms by which leukocytes may regulate coagulation. Further, in vivo studies have provided support for the relevance of these mechanisms in pathophysiologic coagulation.