ABSTRACT
To understand developmental abnormalities including pervasive developmental disorders, it is essential to understand normal developmental process of children, especially from infancy to preschool years. Infancy refers to the time before the beginning of extensive verbal communication that occurs at about 18 months. Toddlerhood, from 18 months to 3 years, encompasses the most rapid and contextually transactional period of developmental change throughout the postpartum life. Preschool years are characterized by extensive development of language, cognition, and social ability including the blooming of theory of mind. Multiple areas of development are affected in pervasive developmental disorders (PDD), and assessments of various aspects of behavior and development are essential for diagnosis. This review provides the areas of assessment of core features of PDD, including abnormalities in communication, social interaction, and repetitive and stereotyped behavior in the context of deviation from the normal behavioral development from infancy to preschool years.