Tension-type headache, the most common type of primary headache disorder, is reclassified in the second International Headache Society classification with clear diagnostic criteria. Chronic tension-type headache (CTTH) differs from episodic form in frequency, lack of response to most treatment strategies, more medication overuse, and more loss of quality of life. New concepts in the pathophysiology of CTTH emphasize the possible role of central nociceptive pathway sensitization in addition to peripheral myogenic factors. Mechanisms of central sensitization, even though poorly understood, may involve nitric oxide system and N-methyl-D-aspartate receptors. Future treatment modalities are likely to be based on such mechanisms. Using MRI and voxel-based morphometry, structural abnormalities have been found in patients with CTTH for the first time. Pain processing areas such as dorsal rostral and ventral pons, anterior cingulate cortex, anterior and posterior insular cortex, right posterior temporal lobe, orbitofrontal cortex, para hippocampus bilaterally, and the right cerebellum were found to have decreased gray matter in patients with CTTH compared with control subjects and patients with medication overuse headache. Amitriptyline remains the most effective preventive treatment so far. The role of botulinum toxin is not fully defined.