Our knowledge of the cognitive and social-emotional functioning of developmentally disabled infants and preschoolers derives, in large part, from our assessment of such children. This book has been developed to familiarize readers with the characteristics of developmentally disabled children, and to introduce to readers aspects of measurement that are of relevance to the assessment of atypical infants and preschoolers. The book has been developed with clinicians and prospective clinicians in mind. These are individuals who are committed to the care and education of developmentally disabled infants and preschoolers and the families of those children. The book has thus been written to provide support for the use of assessment data in planning early interven tion programs. Of special note in the development of this edited book is that it is divided into four major parts with interrelated chapters in each part. The authors of chapters in Parts II and III had access to the chapters in Part I before writing their chapters. The summary chap ters found in Part IV were similarly written by authors having access to all chapters in Parts I-III. This approach to the development of an edited book was chosen as a way of ensuring an integration of major concepts throughout the book. This process is also a reflection of our belief that assessment is an interdisciplinary process, involving the syn thesis of a number of diverse interests.