Under Part C of the Individuals with Disabilities Education Act (IDEA), participating states and jurisdictions must provide services to children who are either experiencing developmental delays, or who have a diagnosed mental or physical condition that has a high probability of resulting in developmental delay. Additionally, states may choose to serve children who are at risk of having substantial developmental delays if early intervention services are not provided. Eligibility criteria used by states influence the numbers and types of children receiving services, the types of services provided, and the cost of the early intervention system. This paper discusses how 50 states and 6 jurisdictions define developmental delay and, as applicable, their approaches to serving children who are at risk. It discusses the wide variability in the types of criteria states use to describe developmental delay and identifies three categories of risk for adverse developmental outcomes used by many states. These include conditions of established risk, biological/medical risk, and environmental risk. Factors that may mediate the impact of risk factors are also noted, including temperament of the child, high self-esteem, good emotional relationship with at least one parent, and successful learning experiences. Definitions related to eligibility under Part C of the IDEA Amendments of 1997 are provided. A summary table lists the following for each state: level of developmental delay required for eligibility; whether or not at-risk infants and toddlers are served; and comments.