This review deals with the properties and functions of dipeptidyl peptidase IV (DPP IV, EC 3.4.14.5). This membrane anchored ecto-protease has been identified as the leukocyte antigen CD26. The following aspects of DPP IV/CD26 will be discussed: the structure of DPP IV and the new family of serine proteases to which it belongs, the substrate specificity, the distribu­tion in the human body, specific DPP IV inhibitors and the role of CD26 in the intestinal and renal handling of proline containing peptides, in cell adhesion, in peptide metabolism, in the immune system and in HIV infection. Especially the latest developments in the search for new inhibitors will be reported as well as the discovery of new natural substrates for DPP IV such as the glucagon-like peptides and the chemokines. Finally the therapeutical perspectives for DPP IV inhibitors will be discussed.