The Assessment of SpondyloArthritis International Society (ASAS) group has recently developed criteria to classify patients with axial SpA with or without radiographic sacroiliitis, and criteria to classify patients with peripheral SpA. The ASAS axial criteria consist of 2 arms and can be applied in patients with back pain (>3 months almost every day). In one arm, imaging (radiographs and magnetic resonance imaging [MRI]) has an important role, in the other arm--HLA-B27. MRI can detect active inflammation and structural damage associated with SpA. According to the ASAS axial SpA criteria, patients with chronic back pain aged less than 45 years at onset can be classified as having axial SpA if sacroiliitis on imaging (radiographs or MRI) plus 1 further SpA feature are present, or if HLA-B27 plus 2 further SpA features are present. The ASAS peripheral criteria can be applied in patients with peripheral arthritis (usually asymmetric arthritis predominantly involving the lower limbs), enthesitis, or dactylitis. Patients can be classified as having peripheral SpA if 1 of the following features is present: uveitis, HLA-B27, preceding genitourinary or gastrointestinal infection, psoriasis, inflammatory bowel disease, sacroiliitis on imaging (radiographs or MRI), or if 2 of the following features besides the entry feature are present: arthritis, enthesitis, dactylitis, inflammatory back pain, or a positive family history of SpA.