A major challenge to state and jurisdictional policy makers in implementing the Early Intervention Program for Infants and Toddlers with Disabilities, Part C under the Individuals with Disabilities Education Act (IDEA), continues to be determining definitions of developmental delay and criteria of eligibility for services to young children, birth through 2 years of age, and their families. Under Part C, participating states and jurisdictions must provide services to two groups of children: those who are experiencing developmental delays, and those who have a diagnosed mental or physical condition that has a high probability of resulting in developmental delay. In addition, states may choose to serve children who are at risk of having substantial developmental delays if early intervention services are not provided. Over the years, several states have revised their definitions: some have narrowed their eligibility criteria and others have expanded them. This paper updates earlier versions of the same title. It provides the most recent information on how the 50 states and 6 jurisdictions that participate in the Part C program define developmental delay and, as applicable, at risk in their definition of eligibility for services.