The spondyloarthritides (SpA) are currently differentiated into axial and peripheral SpA. Patients with axial SpA (axSpA) may be further classified into the classical form ankylosing spondylitis (AS) and non-radiographic axSpA (nr-axSpA). The SpA are genetically linked, and the subtypes including psoriatic arthritis (PsA) share characteristic clinical symptoms such as inflammatory back pain (IBP) and enthesitis. IMP can be due to sacroiliitis and spondylitis, enthesitis may occur with or without arthritis, and anterior uveitis, as well as other extraarticular manifestations such as psoriasis and chronic inflammatory bowel disease (IBD). In addition to clinical findings, imaging, mainly conventional radiography and magnetic resonance imaging (MRI), and laboratory results such as HLA B27 and CRP are important tools for classification and diagnosis of SpA. The Assessment of SpondyloArthritis international Society (ASAS), an international group of experts in the field of SpA since 1995, has published on assessments and outcome parameters in SpA. The publication of classification criteria for axSpA has now largely replaced the 1984 criteria for AS. However, the established cut-off between AS and nr-axSpA, 'definite' structural changes in the sacroiliac joints, has been recently debated because of limited reliability. Since imaging plays an important role in all criteria sets, the ASAS group has recently published definitions for inflammatory changes in the SIJ and the spine. The most important domains in AS are disease activity, function, spinal mobility, structural damage, and quality of life, some of which are discussed in this manuscript. For axSpA there are two major tools to assess disease activity, the BASDAI and the ASDAS, one for function, the BASFI, and several mobility measures including the BASMI. The AS Health Index (AS-HI) is introduced elsewhere in this supplement.