The spondyloarthritides (SpA) are a family of diseases that include ankylosing spondylitis (AS), reactive arthritis (ReA), psoriatic arthritis (PsA), enteropathic arthritis, juvenile-onset SpA, and undifferentiated SpA. More recently, these disorders have been broadly classified as predominantly axial SpA, affecting the spine, pelvis, and thoracic cage, or predominantly peripheral SpA, affecting the extremities. Classification criteria based on these main clinical features have been developed that aide in early identification (Tables 163-1 ASAS Criteria for Classification of Axial Spondyloarthritis (To Be Applied for Pts with Back Pain 3 Months and Age of Onset 45 Years)a and 163-2 ASAS Criteria for Peripheral Spondyloarthritisa ).