This chapter summarizes etiologies of thrombocytopenia not covered elsewhere in this book. Thrombocytopenic disorders discussed include a common cause of thrombocytopenia-pseudothrombocytopenia, as well as rare conditions such as acquired amegakaryocytic thrombocytopenic purpura (AATP). Examination of the patient’s peripheral blood smear is a key first step in evaluating thrombocytopenia. Many conditions can be easily diagnosed, such as thrombocytopenia due to vitamin deficiency, clonal disorders, or massive transfusion, while others (such as AATP) require a bone marrow evaluation for diagnosis. Some thrombocytopenic disorders discussed in this chapter respond to platelet transfusion, while others require a different treatment approach, and pseudothrombocytopenia requires no therapy. Summarizes a classification of the miscellaneous thrombocytopenic disorders reviewed in this chapter.