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Abnormal bleeding may result from defects in platelets, coagulation factors and/or blood vessels. Screening tests for coagulation factor abnormalities include the activated Partial Thromboplastin Time (aPTT) and Prothrombin Time (PT) / International Normalized Ratio (INR). Thrombocytopenia is the most common platelet defect. Qualitative platelet defects may also occur and, in some cases, may be associated with thrombocytopenia. In patients with platelet defects, bleeding time and/or closure times using the platelet function analyzer (PFA-100/200^TM) may be prolonged. Vascular defects may be accompanied by joint hyperflexibility or skin laxity. Effective treatment of hemostatic disorders requires accurate diagnosis and special coagulation testing that may include coagulation factor assays, inhibitor assays, and platelet function tests. An algorithmic approach to diagnosis is beyond the scope of this review.