The pain and functional disability in headache is tentatively presented with respect to the displayed causal relationships, beginning with the nociceptor. The concept is presented that primary deep pain must stem from a pain generator (PG), of which the free C-fiber ending together with the extracellular space constitute the (inflammatory) nociceptor; and that neurogenic pain is a secondary, enhancing measure. Headache is considered to be a result of various PGs in pain-sensitive structures implicated in head locomotion. Headache occurs when nociceptive volleys from either the cephalic or cervical levels traverse the locomotor centres of the head in the upper cervical medulla and are realized as referred pain. Some clinical aspects are presented.