The generally reported prevalence of 0.1% to 0.2% for ankylosing spondylitis in the white population is probably too low, because it is based almost entirely on hospital records. The incidence and clinical presentation of ankylosing spondylitis have not changed during the past few decades. For reactive arthritis, the list of microbes recognized as triggering agents is continuously increasing. Reactive arthritis is divided into urogenic, enterogenic, respiratory tract-associated, and idiopathic arthritides. In addition, several microbial diseases may be accompanied by reactive arthritis, even though the identity of the causative agent is not always known. In the diagnosis of the spondyloarthropathies, definite progress has been made in developing classification criteria. Intensive research is going on to evaluate new therapies, with special attention to the use of antimicrobial agents for the treatment of reactive arthritis.