This monograph, from the Clinics in Developmental Medicine series, reports data from a multidisciplinary research project on the Autism and Language Disorders Nosology Project supported by grants from the National Institute of Neurologic Disorders and Stroke. The goal of the project group was to develop a more quantitatively and clinically sophisticated classification system for preschool and young school-aged children with developmental disorders of communication skills. These disorders are among the most common problems seen by disability teams in the developed world.