Ankylosing spondylitis (AS) belongs to the family of spondyloarthritides that also includes reactive arthritis (Chap. 162. Reactive Arthritis), psoriatic arthritis (Chap. 161. Psoriatic Arthritis), enteropathic arthritis (Chap. 165. Other Musculoskeletal Disorders) juvenile-onset spondyloarthritis, and undifferentiated spondyloarthritis. Classification criteria based on their main clinical features of axial spondyloarthritis (Table 160-1) and peripheral spondyloarthritis (Table 160-2) have been developed that aide in early identification. AS is a chronic and progressive inflammatory disease of the axial skeleton with sacroiliitis (usually bilateral) as its hallmark. Peripheral joints and extra-articular structures may also be affected. Most frequently presents in second or third decade more commonly in men; strong association with histocompatibility antigen HLA-B27.