The question of whether mandibular growth can be augmented and its direction modified in order to correct mandibular retrognathia has already been dealt with in numerous publications. Animal studies (Pertrovic et al., 1981; McNamara, 1987) reveal that it is possible to augment mandibular length in the rat and monkey by placing a hyperpropulsor 24 hours per day throughout the growth period. Short-term studies (Pancherz, 1979; Wieslander, 1984; Lautrou, 1993; Pezzin, 1995), medium-term studies (Legoff, 1999) and long-term studies (Fränkel, 1969, 1997) have confirmed these results in man. However, treatments using the Fränkel function regulator last between 4 and 7 years and are therefore difficult to apply. Furthermore, studies into the long-term impact of the monoblock activator (De Vincenzo, 1991) and the Herbst flip-lock systems (Pancherz and Fackel, 1990; Pancherz, 1997, 1998; Wieslander, 1993) have shown that the short-term mandibular changes tend to disappear. These studies found virtually no difference as regards mandibular growth between the treated patients and a control group from both the quantitative and qualitative (direction of growth) viewpoint. According to Johnston (1986, 1998, 1999), this is not due to relapse: Class II activators are unable to augment mandibular growth. They help avoid the advancement of the maxillary arch and have a strictly dentoalveolar action. Hence, it is impossible to claim that any currently used orthodontic-type treatment can augment and redirect mandibular growth in the long term.