Diagnosing spondyloarthritis (SpA) early in young patients with inflammatory back pain and normal findings on radiographs of the sacroiliac joints (SIJ) remains a challenge in routine practice. Magnetic resonance imaging (MRI) is regarded as the most sensitive imaging modality for detecting early SpA before the radiographic appearance of structural lesions. The recently published Assessment of SpondyloArthritis International Society classification criteria for axial SpA include for the first time a positive MRI demonstrating sacroiliitis as an imaging criterion indicative of SpA together with at least 1 clinical feature of SpA. A systematic and standardized evaluation of the SIJ in patients with SpA showed that MRI has much greater diagnostic utility than documented previously and allowed a data-driven definition of a positive MRI for SpA. Single MRI lesions suggestive of inflammation can be found in the SIJ and the spine in up to one quarter of healthy controls and young patients with mechanical back pain.