Platelet dysfunction, especially acquired forms, are common causes of hemorrhage, especially in association with trauma and surgery. Although the hereditary platelet function defects are generally quite rare, hereditary storage pool disease is common enough to be suspected in an individual, usually a child, with characteristic historical and clinical findings. The acquired platelet function defects, especially those resulting from drugs, are very common and should promptly be suspected in patients developing easy and spontaneous bruising, mild to moderate mucosal membrane hemorrhage, or unexplained bleeding associated with trauma or surgery. The template bleeding time is generally useful as a screening test of platelet function, but a normal template bleeding time, in the presence of a suggestive history, suggestive clinical findings, or in the patient frankly bleeding, is not reliable and platelet aggregation or lumi-aggregation should be done in appropriate clinical situations. The mainstay of therapy for essentially all these defects, if bleeding is significant, is the liberal infusion of appropriate numbers of platelet concentrates. The acquired platelet function defects, of course, should also be managed by attempts to treat or control the underlying disease, if possible, and offending drugs or potentially offending drugs should promptly be discontinued.