The maxilla and mandible give the face its proportions and equilibrium, while remaining indissociable from the skull. The harmony of these structures is established according to two planes of reference: the base of the skull in a relationship of cranio-facial interdependence, the dental arch making the occlusal relationship a decisive factor in the concept of facial equilibrium. This occlusal obligation, criterion of normality and stability, confers all of the originality of orthognatic surgery, which must also take into account the periods of cranio-facial growth and oro-facial functions. The authors describe the modern concept of this surgery, the essential place of orthodontic preparation, and technical innovations such as computer-assisted simulation.