Malocclusion is a developmental deformity which may vary from minor to major deformities of dental or skeletal origin, including systemic syndromic anomalies. It may be limited to the maxillofacial bones or encompass the entire craniomaxillofacial region. Because this book is devoted to surgical reconstruction of the craniomaxillofacial skeleton with the goals of achieving normality of health, function, and facial aesthetics, it is important to review how these skeletal malocclusions arise and are classified. Skeletal malocclusion is a set of human craniofacial morphologic characteristics that either exceed or exhibit deficiency of volume and proportion. It results in an improper relationship of the jaws—a relationship that distorts the normal balance of the face, because of difficulties with dental occlusion and the temporomandibular joints. Conceptually this book focuses on the aggregate effect of abnormal growth and development of the distinct skeletal units of the craniofacial anatomy as they relate to function and physical appearance. When considering this problem theoretically, however, the problems of growth and development break down into the interrelationship between human genetics and the response of the genome to the environmental factors that influence its phenotypic expression.