Oral habits in children is common and affects the occlusion as well as the facial development. Early treatment of these habits is very important to prevent the resulted complications. The most common oral habits among children are:thumb and digit sucking,mouth breathing,tongue thrusting,lip and biting Different appliances were used to stop the oral habits in children and treat the malocclusion. . This book investigates the type of oral habits among children, their effect on occlusion and dental arches.It presents appliances used to treat those habits and malocclusion related to them in children. As well it presents a new treatment modality through investigating the effect of prefabricated functional appliance (trainer for Kids or T4k) on oral habits and their effect on different arch parameters.