Research has found that foster youth who remain in care beyond age 18 are more likely to participate in services and tend to have better outcomes than those who do not. However, not all youth eligible to remain in care beyond age 18 do so. This study examines Illinois, one of the few states that extends care up to age 21, to identify the major factors that influence whether young people remain in care beyond 18. The study involved analysis of administrative data, a statewide survey of caseworkers, focus groups with substitute caregivers and with youth, and site visits to interview court personnel across the state. Findings indicate that strong advocacy within the juvenile court on behalf of foster youth plays a primary role in keeping youth in care. In Illinois, courts supervise all cases of youth in foster care, so once court jurisdiction ends, state care and services irrevocably end as well. By keeping cases open, court advocacy enables youth to continue to remain in care and receive other child welfare services. Court advocacy can also affect retention rates indirectly by exerting an influence on other factors that play a role in foster care decisions regarding keeping foster youth in care. A higher degree of court advocacy is associated with a greater availability of placements and services for older foster youth, more involvement by caseworkers and other adults, more positive attitudes about remaining in care beyond 18, and a greater awareness that, by law, youth may remain in care beyond 18.