This book analyzes and suggests an expansion of Llorens’ developmental theory of occupational therapy, applying these concepts in a final schematic model for use by occupational therapists, occupational scientists, and others involved in occupational tasks, relationships, and activities. The book then uses the International Classification of Functioning in a context of health promotion and disease prevention to relate the expanded theory to psychosocial, cognitive, and sensorimotor correlates in preterm infants and their families in the neonatal intensive care unit and after discharge to the home environment. Last, it provides an NICU infant case illustration on the Developmental Analysis, Evaluation, and Intervention Schedule. The major theme of this book focuses upon expanding the psychological, neurophysiological, and sociological aspects of Llorens’ developmental theory for a person-occupation-environment based practice and research. The book will then correlate these concepts with current terminology from the World Health Organization, and specialized knowledge and skills in the neonatal intensive care unit. This book was published as a special issue of Occupational Therapy in Mental Health.