Conventional radiography is still the first and most important imaging examination in a clinical setting when evaluating a patient with a known or suspected diagnosis of osteoarthritis (OA). In research and clinical trials, it still is a valuable tool for stratifying patients who have OA into different categories for inclusion criteria and eligibility. MRI has become crucial in understanding the natural history of the disease and in guiding future therapies because of its ability to image the knee as a whole organ and to assess cartilage morphology and composition directly and in a three-dimensional manner. The other modalities discussed in this article are valuable additional techniques indicated on a case-by-case basis.