PROJECT SUMMARY The Pediatric HIV/AIDS Cohort Study (PHACS) is a large-scale multi-site network with multiple observational studies aimed to evaluate the long-term effectiveness and safety of population-based interventions to prevent perinatal HIV transmission and HIV progression and inform HIV remission/cure strategies. The PHACS Research Projects have generated and will continue to gather data on health determinants from various sources and via diverse methods including clinical, social, genomic, microbiome, and imaging data. A multitude of concept sheets and sub-studies will therefore be proposed requiring specialized epidemiological and statistical expertise, ranging from efficient study design, causal inference methodology, deep learning algorithms, bioinformatics, and integrative microbiome informatics, to inform data analyses that will be conducted by the PHACS Data Resources Core (DRC). As we continue to add to our extensive PHACS database, new and challenging problems are likely to arise that may necessitate the development of innovative study designs and analytic methods, particularly in the fields of pediatric and perinatal epidemiology. Ongoing training will be required on these and other methodological innovations for PHACS epidemiologists and statisticians. Continuing education on fundamental epidemiological and statistical principles will also be required for all PHACS investigators in order to conduct effective collaborative research. To address these needs and promote valid and efficient pediatric and perinatal HIV research within the Research Projects and any focused Emerging Research Pilots, the PHACS Epidemiological and Statistical Methods Core (ESC) has been established to: (1) provide specialized epidemiological and statistical services, support, and resources; (2) promote the development and application of novel epidemiological and statistical methods; and (3) support continuing education in epidemiological and statistical methods through in-person or web-based training and mentorship of early career investigators.