The spondyloarthropathies comprise a large clinical spectrum of disorders that include well-defined clinical disorders such as ankylosing spondylitis and less well-defined disorders such as undifferentiated spondyloarthritis, which refers to patients with inflammatory back pain not fulfilling classification criteria for ankylosing spondylitis and without radiologic evidence of sacroillitis. These disorders share clinical and genetic features and are linked by their association with HLA-B27 and by the presence of enthesitis as the basic pathologic lesion. The prevalence of the entire group ranges between 0.6% and 1.9%. This article reviews the recent advances in our understanding of the clinical spectrum, pathogenesis, genetics, and management of undifferentiated spondyloarthritis.