The effect of head posture on initial occlusive contacts has been studied extensively by researchers in the basic sciences, dentistry, and physical medicine. The purpose of this paper is to review their theories and propose a new mechanism that attributes the effect of head posture to a change in the upper to lower jaw relationship. This mechanism is referred to as the sliding cranium theory. To understand how head posture alters initial tooth contact or muscle contact position, the arthrokinematics of the occipital-atlantal joint are covered in detail. The implication of the proposed new theory have relevance for dentists concerned with occlusal function and the treatment of temporomandibular joint dysfunction with temporomandibular repositioning, as well as for physical therapists who effect a change in head posture through mobilization procedures and therapeutic exercise. To conceptualize the proposed theory two easily performed tests are described. The sliding cranium theory presents a mechanical model that explains the interrelationship between the head-neck complex and the craniomandibular system in a way that has not been previously done.