Multiple areas of development are affected in pervasive developmental disorders(PDD), and assessments of various aspects of behavior and development are essential for diagnosis. The object of this review is to provide the practical guidelines for the assessment of core features of PDD, including abnormalities in communication, social interaction, and repetitive and stereotyped behavior. It covers the issues of differential diagnosis within and outside the PDD category, standardized diagnostic tools, assessment of intellectual and language functions, and the early diagnosis of PDD in infancy. This guideline also stresses the process of medical and neurological evaluation for proper diagnosis of PDD.