Hemostasis occurs in two steps: platelet plug formation followed by fibrin deposition. Platelet disorders cause incomplete or absent platelet plug formation. Platelets form in bone marrow and have an 8- to 9-day life span. A careful history and physical examination can distinguish between platelets or the coagulation cascade as the cause of deranged hemostasis. Platelet disorders are characterized by mucosal and small vessel bleeding, and petechiae are characteristic. A complete blood count and prothrombin time/international normalized ratio testing reveal most causes. Many drugs can alter platelet production, function, and longevity, but antiplatelet therapy is the most common cause. Other causes include congenital, medication side effects, sepsis, bone marrow suppression, and systemic disease. The platelet count may be low, elevated, or normal. Bleeding time is virtually always prolonged in platelet dysfunction. Therapy for platelet dysfunction depends on the etiology and severity. Platelets are short-lived, and platelet transfusion may be necessary in the unstable or actively bleeding patient. In most other cases, removing the cause or treating the systemic disease will improve hemostasis. Emergency physicians treating patients with abnormal bleeding should understand the coagulation cascade and drug-induced coagulopathy from older and newer agents. Key words: bleeding time, coagulopathy, drug induced, hemostasis, megakaryocyte, mucosal bleeding, petechiae, platelet plug, platelet transfusion, platelets, thrombocytopenia