The term juvenile idiopathic arthritis (JIA) encompasses a group of arthritides of unknown cause that begin before 16 years of age and last for at least 6 weeks. Temporomandibular joint (TMJ) involvement has been described in up to 87% of children with JIA and has been associated with all JIA subtypes. TMJ involvement is difficult to detect clinically. In one study, 71% of JIA patients with active TMJ synovitis were asymptomatic and 63% had normal physical examination results. Moreover, the main growth center of the mandible is located in the condyle, separated from the joint space by only a thin layer of fibrocartilage. This makes mandibular growth vulnerable to arthritic changes, eventually resulting in facial asymmetry and retrognathia. Therefore, early detection and treatment of TMJ arthritis are paramount to preserving motility and preventing deformity. As clinical symptoms and physical examination results are not good markers of TMJ involvement, imaging plays a key role in diagnosis and treatment monitoring. Magnetic resonance imaging is the technique of choice for the study of TMJ arthritis. It has the advantages of displaying both soft tissue and bone, is the most sensitive technique for detecting acute synovitis, and is the only one able to demonstrate bone marrow edema. In addition, it allows the assessment of changes to the joint over time and evaluation of the effectiveness of therapeutic interventions. ©RSNA, 2017.