After a short review of the history of Reiter's diseases, new definitions of what is now called reactive arthritis are proposed. The best definition is based on a multiple-entry set of criteria validated by other members of the spondyloarthropathy group. This definition needs no reference to origin hypothesis and no exclusion criteria. The epidemiology of Reiter's syndrome is also discussed, and the problems due to previously imprecise definitions are emphasized. Clinical features are described and divided into four syndromes: peripheral arthritis, enthesopathic, pelvic and axial, and extramusculoskeletal syndromes; seven predictive factors of long-term evolution are proposed. Differential diagnosis is discussed within and without the spondyloarthropathy group.