PROJECT SUMMARY/ABSTRACT Over 80% of children diagnosed with cancer become long-term survivors; however, 70% develop chronic or life-threatening late effects from treatment, and these often emerge during young adulthood. Guidelines recommend annual long-term follow-up care (LTFU) to manage and monitor for late effects, recurrence, or new cancer(s). Yet, as risk for late effects increases in young adulthood and survivors transition into adult-focused care, engagement in care plummets. Disengagement from LTFU leaves adolescent and young adult survivors (AYA) vulnerable to delayed or poorly managed diagnoses, and relates to lower knowledge and self- management abilities. Thus, interventions targeting re-engagement of AYA are critical for this vulnerable population. In order to address this critical intervention need, we propose REACH (Re-Engaging AYA survivors in Cancer-related Healthcare)—an innovative sequential multiple assignment randomized trial (SMART) with a two-stage design. We will leverage our research on transition to adult care, survivorship care plans, and digital health interventions. Stage 1 tests a low touch intervention (LTI), consisting of reminder “nudge” text messages and informational resources for up to 4 weeks compared to an enhanced usual care group that will receive written information (WI) only. Next, in Stage 2, AYA will be re-randomized based on responsiveness to Stage 1 (i.e., whether or not they made an appointment) into 16 weeks of intervention. Responders will either be re- randomized to receive a continued intervention (maintenance) or a stepped-up condition, while non-responders will only receive a stepped-up condition. Step-up may include expanded LTI (more text messages) or a high touch intervention (HTI). The HTI includes text messages and digital resources, including a personalized survivorship care plan (SCP), on a mobile-friendly platform with information tailored to variables such as barriers to care, treatment history and recommendations, and a personal health goal. Additionally, social worker and/or nurse support will be available to help overcome barriers (e.g., access care). The intervention options in Stage 2 are intended to enhance self-management beyond simply attending LTFU in order to sustain long-term engagement. Outcomes are measured after Stage 1 (T2), Stage 2 (T3) and at 9 months (T4). We expect those who start with LTI versus WI will be more likely to attend a LTFU appointment by T4. For Stage 2, those who receive the stepped-up condition compared to those with maintenance, and those who received HTI compared to LTI, will be more likely to attend an appointment by T4. Attending an appointment by T3 and indices of self-management are secondary outcomes. We will also assess moderators of intervention outcomes, intervention engagement, acceptability, feasibility, and cost. Qualitative interviews will assess multilevel barriers and facilitators of future implementation with stakeholders (AYA, parent...