ABSTRACT The Harvard-wide pediatric HSR Fellowship is a 30 year collaboration of Boston Children’s Hospital, Massachusetts General Hospital for Children, and the Harvard Department of Population Medicine/Harvard Pilgrim Healthcare Institute. This program has an amazing training record- the 154 alumni include 18 graduates from race/ethnicity groups underrepresented in medicine, 61 in leadership positions including 16 CMOs or Senior VPs, and four elected to the National Academy of Medicine. The curriculum is grounded in rigorous child health services research training and in-depth implementation science training, aligned with AHRQ learning health system (LHS) competencies. While our multisite collaborative leadership structure has remained constant for two decades, our list of multidisciplinary, innovative faculty is updated regularly. Pediatric HSR training programs are needed because critical epidemiologic, fiscal, and workforce differences, such as a mental health crisis or a relative dearth of quality measures and interventions, set pediatrics apart from adult medicine. Our overarching aim remains: to provide outstanding, comprehensive training across the spectrum of pediatric health services research methods that both advances understanding of equitable health and healthcare systems, and tests and evaluates implementation of novel approaches that demonstrably improve child health. Toward this aim, the objectives for this fellowship are to: 1) recruit and retain diverse candidates; 2) provide exemplary training in health services research and implementation science within LHS; 3) support our dedicated faculty; 4) continuously improve the training program and update the curriculum; and 5) continue to develop diverse graduates to lead the nation in innovative pediatric health services research, including research that accelerates the development of LHS. To meet this goal, training focus areas include: cutting edge and emerging methods in health systems research; application of implementation science and related methods; LHS competencies including embedded research and health equity research within health systems; and patient and family-centered research. The program will support 7 postdoctoral trainees per year for the 2-year postdoctoral fellowship to create a critical mass of trainees across subspecialties, areas of research, and methodological domains. In addition to conducting closely mentored research, each trainee will complete coursework from the Harvard T.H. Chan School of Public Health for core skills of biostatistics and epidemiology and participate in advanced health services research methods training. Over the past 30 years, our program has evolved in response to feedback from trainees, faculty, and our advisory board. In this cycle, we will expand LHS training consistent with AHRQ’s new health equity and embedded research competencies and add an alumni mentoring program. Renewal of our T32 Program will address a critical deficiency i...