THE TERM HEADACHE commonly denotes head pain from brow level up. This outline defines headaches somewhat broadly; it covers both painful and nonpainful discomforts of the entire head, including the face and upper nucha. Since so much that a man describes as headache may be any abnormal head sensation, it is essential for proper treatment to determine whether the complaint is actually one of pain. A useful scheme for the classification of the varieties of headache is one based on pain mechauisms. The divisions rest on experimental and clinical data, together with reasonable inference; the story is far from complete. Yet the arrangement can serve as a framework for diagnostic criteria for the major clinical types of headache and by emphasis on basic mechanisms it offers a logical approach to the planning of therapeutic trials. For convenience, short and simple names are suggested for certain major entities and are indicated in boldface type. Essential in the study of headache in most instances is an appraisal of its close link to the patient’s situation, activities, and attitudes. Sometimes in obvious ways, more often in subtle ones, headache may be the principal manifestation of temporary or sustained dif6culties in life adjustment. These relationships are notably evident in Croups I through V.