Disorders of development include a range of problems first evidenced in childhood. Although most disorders have their origins in childhood, a few fully express themselves before early adulthood. This chapter describes the nature, assessment, and treatment of the more common disorders that are revealed in a clinically significant way during a child’s developing years. The disorders of development affect a range of functioning, from single skills deficits to more pervasive problems that negatively impact a child’s ability to function. Included is coverage of several disorders usually diagnosed first in infancy, childhood, or adolescence, including attention-deficit hyperactivity disorder, oppositional defiant disorder, conduct disorder, learning disorders, communication and related disorders, pervasive developmental disorders (including autistic disorder and Asperger disorder), and intellectual disabilities. Recommendations for future research on the potential for advancing knowledge regarding spectrums within some of these disorders, as well as recommendations for treatment, are outlined.