Purpose: Most adolescent and young adult (AYA)-aged childhood cancer survivors develop physical and/or psychosocial sequelae; however, many do not receive long-term follow-up (LTF) critical for screening, prevention, and treatment of late effects. To develop a health services research agenda to optimize care models, we conducted qualitative research with LTF providers examining existing models, and successes and challenges in maintaining survivors' connections to care across their transition to adulthood. Methods: We interviewed 20 LTF experts (MDs, RNs, social workers, education specialists, psychologists) from 10 Children's Oncology Group-affiliated institutions, and analyzed data using grounded theory and content analysis techniques. Results: Participants described the complexity of survivors' healthcare transitions. Survivors had pressing educational needs in multiple domains, and imparting the need for prevention was challenging. Multidisciplinary LTF teams focused on prevention and self-management. Care and decisions about transfer were individualized based on survivors' health risks, developmental issues, and family contexts. An interplay of provider and institutional factors, some of which were potentially modifiable, also influenced how transitions were managed. Interviewees rarely collaborated with community primary care providers to comanage patients. Communication systems and collective norms about sharing care limited comanagement capacity. Interviewees described staffing practices, policies, and informal initiatives they found reduced attrition. Conclusions: Results suggest that survivors will benefit from care models that better connect patients, survivorship experts, and community providers for uninterrupted LTF across transitions. We propose research priorities, framing attrition from LTF as a public health concern, transition as the central challenge in LTF, and transition readiness as a multilevel concept.