Muscle fatigue has been thought to be one of the causes of pain associated with temporomandibular disorders. A multitude of variables could contribute to neuromuscular fatigue when a subject attempts to sustain a given force. In studies of jaw muscles the endurance limit has been related to a failure in electrical conductivity (transmission fatigue), an increasing imbalance in the intracellular contents of muscle fibers (contraction fatigue), and the onset of pain. This review describes the principles that underlie fatigue and the results of studies of jaw muscle fatigue. Attempts are made to explain why various studies may have produced different results.