This paper presents the answers of Psychology to three urgent tasks related to children at risk, namely, a theoretical comprehension on the origins of disorders, diagnosis procedures and treatment procedures for children with disorders. To attain these goals, the paper explains the concept of deviated development and the contributions of Applied Behavior Analysis (ABA) for diagnosis and training of children at risk, as well as the work made by the author with slum children; some cases are shown. It is concluded that ABA techniques can be applied in the rehabilitation, training and special education of children with intellectual, sensorial or motor disabilities; these techniques are efficient and can be applied successfully by non professional persons. Finally, the paper proposes a model to teach Psychology in the special conditions of developing countries, uniting teaching, research and community service.