Aim: This article demonstrates the significant impact of treatment timing on the outcome of functional jaw orthopedics for Class II and Class III malocclusions. Methods: Subjects with Class II and Class III malocclusions treated with functional appliances and face mask therapy, respectively, were analyzed using matched samples of untreated subjects as control groups. Both experimental and control groups were divided according to maturational stages. In particular, the cervical vertebral maturation method was applied to discriminate between subjects who started functional therapy of Class II malocclusion before the peak in mandibular growth velocity (early treatment group) and those who started during or slightly after this peak (late treatment group). Results: Both active treatment and posttreatment results of rapid maxillary expansion and face mask therapy for Class III malocclusions indicate that correction of the skeletal disharmony primarily depends on the amount of beneficial changes obtained during active therapy. The earlier the correction (ie, in the primary or early mixed dentitions), the greater the final favorable skeletal change in both the maxilla and the mandible. In Class II therapy, treatment with functional appliances during or slightly after the pubertal peak in growth velocity induces a supplementary elongation of the mandible that is more than twice that observed when treatment is performed before the peak. Conclusion: Ideal treatment timing for the Class III malocclusion is in the early mixed dentition, whereas it is in the circumpubertal period for the Class II malocclusion associated with mandibular deficiency. The cervical vertebral maturation method, a simple and reliable method for detection of the pubertal peak in skeletal growth of the individual patient, is described. World J Orthod 2001;2:159-167.