This article discusses why and how states are integrating "DC:0-5™: Diagnostic Classification of Mental Health and Developmental Disorders of Infancy and Early Childhood" (DC:0-5) into state policy and systems; provides state examples to highlight some of the strategies that states have used to allow, promote, or require the use of DC:0-5; and provides recommendations for further improvements in state infant and early childhood mental health policy and practice. The article provides a point-in-time overview of how DC:0-5 is being integrated into state systems and policies and is a sample of the innovative work going on across the United States. The article is adapted from a paper of the same name published in 2018 by the ZERO TO THREE Policy Center.