 There are numerous publications comparing the activators effects or the appliance effects on mandibular growth, but none have studied the Twicare. The aim of this study is to assess the efficacy of this specific removable appliance in mandibular advancement in Class II skeletal in children aged from 7 to 12 years old. The appliance will be compared with the Herbst and one untreated group will be included to control the internal validity of the study as recommend in noninferiority trials. 