This article introduces the revised and updated "DC:0-5™: Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood." The authors describe the past and current efforts to create a developmentally based classification system for very young children. DC:0-3, published in 1994 by ZERO TO THREE, was created to address the significant need for a systematic, developmentally based approach to the classification of mental health and developmental difficulties in the first 4 years of life (i.e., birth through 3 years old). Based on advances in knowledge, ZERO TO THREE decided to revise DC:0-3 and published DC:0-3R in 2005. The current revision, DC:0-5, was substantial, expanding the age range from 3 years old to 5 years old and expanding the number of diagnostic categories and clinical disorders from previous versions. The authors describe the new features of DC:0-5, detail three levels of the training model, highlight other DC:0-5 resources, and discuss the future of DC:0-5.