The history of classification and diagnostic criteria for rheumatoid arthritis (RA) and ankylosing spondylitis (AS) is similar and different. Important criteria sets have been published for both disease in the mid eighties, for AS in 1984 and for RA in 1987. The leading clinical symptoms, inflammatory back pain (IBP) in AS and the predominant polyarticular symmetric involvement of the hands in RA were, of course, central, and so was morning stiffness as a major clinical sign of an inflammatory disease state. In RA, there was more focus on laboratory parameters (rheumatoid factor), while this could have been the case also in AS (HLA B27) but this was not recognized at this point in time. In contrast, imaging has played a more important role in AS - especially because the sacroiliac joints are involved in the vast majority of AS patients, while in RA radiographic changes of the joints of hands and feet may contribute to the diagnosis. However, in both diseases, early structural changes visualized by conventional radiography rather have prognostic impact since these patients are much more likely to progress in comparison to others who do not have cartilage and joint damage early in the course of the disease. Further developments of criteria for AS have broadened the spectrum of AS to spondyloarthritis (SpA) and axial SpA which covers most early forms. The leading clinical symptom is chronic back pain in young adults and IBP. New criteria for RA which include more patients with early disease and anti-CCP antibodies as new markers are being developed. This is important since early treatment strategies are increasingly and successfully used to treat inflammatory diseases more efficiently.