A REALISTI c approach to prevention and interception of malocclusion requires an understanding of the basic concepts of etiology. It has been known for some time that heredity plays a vital role; however, sometimes this role is viewed out of proportion and there is a failure to heed the local and environmental factors which come into play. Any consideration of the genetic potential of the dental-facie-skeletal pattern must recognize that this genetic potential runs the gamut of bizarre and incalculable environmental hazards. In many instances the environmental factors are restrictive and prevent the genetic potential from ever being fully reached. On the other hand, without adequate environmental support the genetic potential also fails to reach its full limit. An example of this may be seen in cases of aglossia where lack of tongue function incompletely augments the full development of the dentition. Genetic factors, then, can do no more than govern the norm of reaction of the dentition to its environment. Conversely, it might be said that the environment alters the expression of the genetic factors. We must not consider heredity and environment as necessarily competing or antagonistic forces, but rather as being interwoven, each modifying the other, and their interaction determining the state of occlusion. The developing dental occlusion is dependent upon: 1. Growth of the supporting bone. 2. Size and shape of the developing teeth. 3. Directional development and eruption of these teeth. 4. Interaction and balance of forces operative upon the teeth and the alveolar processes. Whereas the first two areas have a high correlation with genetic factors, the latter two are intimately related to a vast complex of interacting local factors. These may relate to the presence of interferences such as supernumerary teeth, cysts, or over-retained and ankylosed teeth. Traumatic injury to deciduous teeth commonly deflects permanent erupting successors. Other factors are missing teeth, premature loss of teeth (with drifting), interproximal caries, improper fillings, peculiar shape and size of contacting teeth and the presence of a frenum. Operative also is the possible imbalance of the supporting musculature, hyperhypotonic oral muscles, thick or heavy buccinator muscles, short or hypertonic upper lips, or numerous variations in tongue size and shape, and peculiarities