Von Willebrand factor cleaving protease was first identified in 1987 and was further classified several years later as ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin-1-like domains). Congenital and acquired deficiency of ADAMTS-13 is associated with thrombotic thrombocytopenic purpura (TTP) and other thrombotic microangiopathies (TMAs). Assays for measurement of ADAMTS-13 were developed in the late 1990s, and significant improvements have occurred in the testing protocols to allow them to be performed in routine hemostasis laboratories. This article reviews the original ADAMTS-13 activity assays and those currently available. It also reviews the consistency of results among various methods and discusses the clinical utility of ADAMTS-13 testing in TTP, TMA, and other disease conditions.