A bleeding disorder may be suspected when a patient reports spontaneous or excessive bleeding or bruising, often secondary to trauma. Possible causes can vary between abnormal platelet number or function, abnormal vascular integrity, coagulation defects, fibrinolysis, or a combination thereof. This review addresses hemorrhagic disorders associated with quantitative or qualitative platelet abnormalities, such as thrombocytopenia, platelet function disorders, thrombocytosis and thrombocythemia, and vascular purpuras. Hemorrhagic dis­orders associated with abnormalities in coagulation (e.g., von Willebrand disease and hemophilia) are not covered. An algorithm shows evidence-based practice guidelines for the management of immune thrombocytopenic purpura. Tables list questions regarding bleeding and bruising to ask patients, clinical manifestations of hemorrhagic disorders, typical results of tests for hemostatic function in bleeding disorders, causes of thrombocytopenia, other forms of drug-induced thrombocytopenia, classification of platelet function disorders, and selected platelet-modifying agents. This review contains ­1 highly rendered figure, 7 tables, and 82 references.