The tenderness of pericranial myofascial tissues and number of myofascial trigger points are considerably increased in patients with tension-type headache (TTH). Mechanisms responsible for the increased myofascial pain sensitivity have been studied extensively. Peripheral activation or sensitization of myofascial nociceptors could play a role in causing increased pain sensitivity, but firm evidence for a peripheral abnormality still is lacking. Peripheral mechanisms are most likely of major importance in episodic TTH. Sensitization of pain pathways in the central nervous system due to prolonged nociceptive stimuli from pericranial myofascial tissues seem to be responsible for the conversion of episodic to chronic TTH. Treatment directed toward muscular factors include electromyography biofeedback, which has a documented effect in patients with TTH, as well as physiotherapy and muscle relaxation therapy, which are most likely effective. Future studies should aim to identify the source of peripheral nociception.