Thrombotic thrombocytopenic purpura (TTP) is characterized by microangiopathic hemolytic anemia and thrombocytopenia, accompanied by microvascular thrombosis that causes variable degrees of tissue ischemia and infarction. Intravascular coagulation is not a prominent feature of the disorder. Plasma exchange can induce remissions in approximately 80% of patients with idiopathic TTP, but patients have a much worse prognosis when thrombotic microangiopathy is associated with cancer, certain drugs, infections, or tissue transplantation. Recently, acquired autoimmune deficiency of a plasma metalloprotease named ADAMTS13 was shown to cause many cases of idiopathic TTP. This review describes our current understanding of how to use this knowledge clinically. In Section I, Dr. Joel Moake describes the presentation of thrombotic microangiopathy, emphasizing the pathophysiology of idiopathic TTP. Platelets adhere to ultra-large (or "unusually large") von Willebrand factor (ULVWF) multimers that are immobilized in exposed subendothelial connective tissue and secreted into the circulation in long "strings" from stimulated endothelial cells. ADAMTS13 cleaves ULVWF multimers within growing platelet aggregates under flowing conditions, and this normally limits platelet thrombus formation. If ADAMTS13 is absent, either congenitally or due to acquired autoantibodies, platelet-rich microvascular thrombosis proceeds unchecked and TTP ensues. Plasma exchange is effective therapy for idiopathic TTP, probably because it replenishes the deficient ADAMTS13 and removes some of the pathogenic autoantibodies and endothelial-stimulating cytokines. Some patients have a type of thrombotic microangiopathy after transplantation/chemotherapy but do not have severe ADAMTS13 deficiency. The pathogenesis of their disease must differ but remains poorly understood. In Section II, Dr. Toshiyuki Miyata describes recent advances in assay methods that should facilitate routine laboratory testing of ADAMTS13 for patients with thrombotic microangiopathy. ADAMTS13 cleaves a single Tyr-Met bond in domain A2 of the VWF subunit. ADAMTS13 assays based on the cleavage of plasma VWF multimers have been used extensively but require considerable time and expertise to perform. A recombinant substrate containing 73 amino acid residues of VWF domain A2 has been devised that allows short incubation times and rapid product detection by gel electrophoresis or immunoassay. These results should encourage the development of even simpler assays that can be performed in most clinical laboratories. In Section III, Dr. James George provides an update on the long-term prospective study of thrombotic microangiopathy in the Oklahoma TTP-HUS Registry. At presentation, the clinical distinction between idiopathic TTP, various forms of secondary thrombotic microangiopathy, and even Shiga toxin-associated hemolytic uremic syndrome (HUS) can be problematic because the symptoms and laboratory findings often overlap. Consequently, plasma exchange usually is administered to any patient with thrombotic microangiopathy if there is doubt about the cause. The role of ADAMTS13 testing in choosing therapy remains uncertain, but the results do appear to have prognostic significance. Severe ADAMTS13 deficiency is specific for idiopathic TTP and identifies a subgroup with a high likelihood of response to plasma exchange, and high-titer ADAMTS13 inhibitors correlate strongly with a high risk of relapsing disease. Patients with normal ADAMTS13 activity have a much worse prognosis, although many factors probably contribute to this difference. Longitudinal study of these patients will continue to clarify the relationship of ADAMTS13 deficiency to the clinical course of thrombotic microangiopathy.