ADAMTS13, encoded on chromosome 9q34, is a member of the ADAMTS (a disintegrin and metalloprotease with thrombospondin type 1 motif) metalloprotease family, containing the common domain structure of (from the amino terminus) signal peptide, propeptide, reprolysin type metalloprotease, thrombospondin type 1 motif, cysteine-rich region, and spacer domain. ADAMTS13 cleaves von Willebrand factor (VWF) in a shear stress dependent manner. Deficiency of the enzyme causes the platelet aggregation of thrombotic thrombocytopenic purpura (TTP). Inhibitory antibodies of ADAMTS13 are detected in patients with acquired TTP, while homozygous or double heterozygous mutations of the ADAMTS13 gene cause the hereditary form of the disease 1. Targeting of the ADAMTS13 gene by recombinant technology has reproduced the phenotype of human TTP in ADAMTS13-null mice 2. Despite these advances, intense controversy and confusion persist regarding the role of ADAMTS13 assays in the diagnosis of TTP. This brief review highlights some of the contentious issues and proposes steps to improve the diagnostic value of ADAMTS13 assays.