History, theories, methods and findings are characteristic of all disciplines. Also characteristic is the tendency to keep within a discipline much of what is known from discovery by scholars in other fields. While unintentional for the most part, the scant knowledge sharing across disciplines arises from deficits or uncertainties in understanding the definitions of terms, references to key constructs and features, variations in methodologies, and in interpretations of findings. Yet, as the chapters of this book reveal, much is known across several disciplines that converge on many of the same developmental disorders that are (and have been for decades) the focus of considerable research activity. The extensive interest in the study of developmental disorders exists both to understand their natures and causes as well as to reveal differences and similarities in development across normal and abnormal populations.