Thrombocytopenia is defined as a platelet count less than 150,000/μL. It can be the result of decreased platelet production, sequestration of the platelets, or increased destruction of the platelets. The clinical presentation may vary from an incidental finding to obvious bleeding. Causes of thrombocytopenia include infections, malignancy, liver disease, autoimmune disorders, disseminated intravascular coagulation, pregnancy, medications, and coagulation disorders. Treatment is determined by the underlying cause of the thrombocytopenia. This article discusses the evaluation and management of common causes of thrombocytopenia.