The hallmark of ankylosing spondylitis (AS) is acute and chronic spinal inflammation initiating in the sacroiliac joints, often coupled with enthesitis, presenting as chronic inflammation at the sites of ligamentous and tendinous insertions into bone. Peripheral joint synovitis can be a prominent feature as well. Reactive arthritis (ReA) is a sterile synovitis arising after enteric or urogential tract infections. A great progression has been recently achieved in revealing the causes, and making plans for the treatments for AS and other types of spondyloarthropathy. The human leukocyte antigen (HLA)-B27 has been well known to be associated with disease susceptibility in AS and ReA. But the pathogenesis of AS and ReA is still not well defined. Although the clinical manifestations of AS and ReA may differ, in this review we discuss the two diseases together and focus on recent developments on the pathogenesis of both diseases.