Currently temporomandibular disorders (TMDs) can be reliably diagnosed based on symptoms and findings specifically associated with the masticatory system.(1) One would expect that the response of TMD patients to therapy would be proportional to the severity of the TMD as assessed by these symptoms and findings. However, published studies have indicated quite the opposite; response to a treatment targeted for the physical condition is not strongly predicted by the symptom status or severity of the findings. Moreover, published studies indicate that TMD treatments that are targeted to the person (eg, behavioral treatment) often are at least as, if not more, effective as treatments targeted to the physical condition. Therefore, it seems that treatment responsiveness for TMD patients is determined by a wide range of factors beyond those local or specific to the masticatory system. Why these factors and in particular their interactions are so important for predicting TMD treatment response might be better understood by starting with the etiology of the particular disorder.