California's Juvenile Justice Crime Prevention Act (JJCPA) funds the implementation of programs for justice system--involved or at-promise youth (i.e., youth who are at risk of becoming involved in the justice system). In this report, the authors evaluated the implementation of programs run by three Los Angeles (L.A.) County agencies that receive JJCPA funds: the Department of Public Health (DPH), the L.A. County Library, and the Department of Beaches & Harbors (DBH). DPH programs help youth address and treat substance use issues, the Library Probation Outreach Program provides library resources and life skills, and the DBH WATER Youth Program offers ocean safety education and recreation. Although the programs vary in terms of content and setting, they each provide prosocial activities, link youth to resources, and aim to prevent future justice-system involvement. The authors found that these shared characteristics -- along with dedicated and skilled staff, tailored programming, and partnerships with other agencies or organizations -- facilitated program implementation. However, the authors also found common barriers to implementation. All programs faced challenges with attendance. Some programs faced staffing challenges, slow hiring and approval processes, and billing issues. The authors make recommendations to improve implementation to better enable the programs to further their missions and expand their influence on the lives of youth they serve. [This report was prepared for Los Angeles County Probation Department by the Justice Policy Program within RAND Social and Economic Well-Being.]