There is a very substantial literature over the past 50 years on the advantages of early detection and intervention on the cognitive, communicative, and social-emotional development of infants and toddlers at risk for developmental delay due to premature birth or social disadvantage. Most of these studies excluded children with severe delays or other predisposing conditions, such as genetic or brain disorders. Many studies of children with biological or sociodevelopmental risk suggest that behavior disorders appear as early as 3 years and persist into adulthood if not effectively treated. By contrast, little is known about the infants and toddlers with established risk for severe delays who make up a significant proportion of the population with dual diagnoses later in life. In the past decade, there has been a growing interest in early detection and intervention with children age birth to 3 years, for example, the Birth-Three population (Individuals with Disabilities Education Act, 1997), who may have disabilities and severe behavior problems, for example, aggression, self-injury, and repetitive stereotyped behaviors. The available research is scattered in the behavior analytic literature and in the child development literature as well as in the child mental health and psychiatry literature, the developmental disability literature, the animal modeling literature, and the genetics literature. The goal of this introductory overview is to integrate these literatures, by cross-referencing members of these various groups who have worked in this field, in order to provide the reader with an integrated picture of what is known and of future directions that need more research.