In this article, the short- and long-term results of two types of functional appliances are discussed regarding their ability to stimulate the mandibular growth at mandibular retrognathia and reduce an increased overjet. Removable functional appliances, or activators, are compared to a fixed functional appliance, the Herbst appliance. The activator, often consisting of an acrylic base, is advised to be worn for 12 to 20 hours a day. The Herbst appliance consists of interconnected bands around the molar- and premolar bands, keeping the mandibula continuously positioned forward by means of hinges or telescopes. In the short-term, both appliances are effective in reducing the overjet, improving the molar-occlusion and reducing the mandibular retrognathia. The comparative literature is inconclusive as to which appliance is more effective on which level, skeletal or dentoalveolar. The removable appliances are more likely to be accepted at a younger age, whilst the fixed appliances are more suitable for the adolescents. The stability of the long-term treatment effects is minimally described in the existing literature. However, the highest stability rate seems to apply to the Herbst appliance. The impact of a widely applied second phase of treatment with fixed appliances, with possible use of intermaxillary class II elastics and retention using functional appliances is barely taken into account.