The Assessment in SpondyloArthritis international Society (ASAS) defined new criteria in 2009 for the classification of axial spondyloaithritis (SpA) in patients with >= 3 months of back pain who were aged <45 years at the onset of back pain. This represents a culmination of a number of efforts in the last 30 years starting with the 1984 modified New York criteria for ankylosing spondylitis, followed by the 1990 Amor criteria and the 1991 European Spondyloarthropathy Study Group criteria for SpA. The importance of new ASAS criteria for radiologists is that magnetic resonance imaging (MRI) takes center stage and is one of the major criteria for the diagnosis of axial SpA when active (or acute) inflammation is present on MRI that is highly suggestive of sacroiliitis associated with SpA. According to the new criteria, sacroiliitis on imaging plus >= 1 SpA features (such as inflammatory back pain, arthritis, heel enthesitis, uveitis, dactylitis, psoriasis, Crohn's disease/colitis, good response to non-steroidal anti-inflammatory drugs, family history for SpA, HLA-B27 positivity, or elevated C-reactive protein) is sufficient to make the diagnosis of axial SpA. A number of rules and pitfalls, however, are present in the diagnosis of active sacroiliitis on MRI. These points are highlighted in this review, and a potential shortcoming of the imaging arm of the ASAS criteria is addressed.