Reviews factors in the development of normal and abnormal interocclusal clearance. Discusses importance of abnormal clearance in cephalometric analysis and in the differential diagnosis, treatment planning of overbite, and the treatment of malocclusion involving mesial functional mandibular displacement. Outlines treatment of Class III malocclusion involving alterations in the position and growth direction of the mandible with the temporary creation of a large interocclusal clearance, and a simple method of preorthodontic guidance in certain Class II and Class III malocclusions.