Earlier definitions of migraine and tension-type headache assumed knowledge of pathophysiology. These definitions were system specific and simplistically defined migraine as vascular in origin and tension-type headache as muscular. These definitions were vague and have led to lack of precision in diagnosis. Newer understandings of the pathophysiology of headache suggest that there may be a relationship between the two disorders based on central mechanisms. Overuse of analgesics and ergotamines has also been found to contribute to chronic daily headache through the phenomenon of rebound pain. These mechanisms have important implications in the treatment of primary headache disorders.