Division TEACCH, located in the School of Medicine at the University of North Carolina at Chapel Hill, was one of the first programs in the country to understand that autism was an organic rather than a biologic condition. We were also one of the earliest programs to recognize the enormous variability in characteristics and behaviors of children described as autistic. For these reasons, the processes of diagnosis and assessment have always been important and central to our program. We are therefore extremely pleased to have a volume representing the most current thinking of the field's leaders in these important areas. As with the preceding books in our series, Current Issues in Autism, this volume is based on one of the annual TEACCH conferences held in Chapel Hill each May. The books are not simply published proceedings of the conference papers, however. Rather, conference participants are asked to develop a full chapter around their presentations. Other international experts whose work is beyond the scope of the conference, but related to the major theme, are asked to contribute chapters as well. These volumes are designed to provide the most current knowledge in research and professional practice available on the most important issues defining and clarifying autism.