The expansion of early childhood services has brought increasing recognition of the need to address mental health disorders in young children. The transactional perspective of developmental psychopathology is the basis for review of diagnostic frameworks for young children. The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) is discussed in light of developmental and contextual factors that characterize emotional and behavioral problems in infants, toddlers, and preschool age children. The Diagnostic and Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood (DC:0-3) system, developed by Zero to Three: The National Center for Clinical Infant Programs, is introduced as an alternative framework offering developmentally appropriate diagnoses and incorporating aspects of family functioning. Research using the DC:0-3 diagnostic system is reviewed, followed by a discussion of applications of this framework for school psychologists working in early childhood programs.