Although the diagnosis and structural monitoring of ankylosing spondylitis relies classically on standard radiography, recent classification criteria and recommendations issued by the ASAS-OMERACT working group give considerable weight to modern imaging methods, most notably magnetic resonance imaging (MRI). MRI and, more recently, ultrasonography, yield three major benefits: they ensure the early diagnosis of ankylosing spondylitis in the absence of radiographic sacroiliitis, they provide therapeutic guidance at any time during the course of the disease, and they supply objective information on the degree of inflammation and response to treatment. Prospective longitudinal studies are under way to determine the respective roles for MRI and ultrasonography in the diagnosis and monitoring of axial and peripheral forms of ankylosing spondylitis. The introduction of whole-body MRI, new MRI sequences and positron emission tomography can be expected to further benefit the diagnosis of ankylosing spondylitis.