New classification criteria for axial spondyloarthritis (SpA) have been developed and validated. MRI of the sacroiliac joints is an important feature in these criteria. This is rightfully so, as MRI can identify active lesions in the subchondral bone marrow that are thought to be related to the underlying pathophysiological process at the cartilage-bone junction. Follow-up studies using various imaging techniques, including MRI, in unselected patients with undiagnosed back pain of short symptom duration will provide more information on the differential diagnostic capacity of MRI and its predictive value for long-term outcomes. Even longer-term follow-up (>10 years) is necessary to provide reliable data; however, the validity of MRI in the diagnostic process can only be approximated, as it will never cover the entire gestalt of SpA.