It should be iterated that this is an overview of pediatric disorders of development and associated behaviors. It is not meant to explore this large and complex topic comprehensively. Additionally, the editors and some of the other chapter authors recognize that there is no international agreement on terminology, and they are careful to point out the differences. There are several examples of the effort to clarify these confusing conflicts. Internationally it is more common to see the term "mental retardation," as opposed to "learning disability," which is the common phrase in the United Kingdom. In the United States, "learning disability" refers to a specific problem of knowledge acquisition within an overall picture of normal intelligence. It can be a maddening discussion, but it is nevertheless necessary for the international community of clinicians, researchers, and authors to communicate effectively.