A common request to Centers such as ours is for information about the prevalence and incidence of youngsters' problems. The intent of this report is to provide a synthesis of the best data and to clarify the limitations of what has been gathered so far. As you will see, available data continue to be quite limited. The synthesis reported here highlights the limitations and underscores major gaps that need filling. It is clear that a great deal more research is needed, and it must be pursued with sufficient resources to enhance and refine the methodology used. At the same time, we all will continue to draw on what has been reported as an essential aid in planning and decision making. In doing so, it is imperative to use that data carefully and wisely-with a full appreciation of its limitations.