Clinical problems and pain involving the masticatory musculature and the temporomandibular joint (TMJ) are embraced in the term temporomandibular disorders (TMD). To treat these patients, clinicians need accurate diagnostic methods. The Research Diagnostic Criteria for TMD (RDC/TMD) have been proposed as a reliable diagnostic system of TMD patients. The system is currently used worldwide by dentists who treat patients with TMD pain. It is based on the patient history and clinical criteria; in some diagnostic subgroups, imaging techniques such as magnetic resonance imaging (MRI) can be included. Although MRI of the TMJ has been used for many years, its diagnostic and therapeutic efficacy—how well MRI supports clinicians in their diagnoses and treatment decisions—is unclear. A systematic review found that the evidence grade for diagnostic efficacy expressed as sensitivity, specificity, and predictive values was insufficient. No studies that investigated the usefulness of MRI for treating TMD patients were found. Several studies have compared clinical TMD diagnoses made using the RDC/TMD with MRI findings of the TMJ; agreement between these two methods was fair to poor. This emphasizes the need for high-quality studies on the diagnostic efficacy of MRI. Referral criteria for MRI are uncertain, except when surgical treatment is being considered as an option.