Purpose: HAPPEN is a primary school national cohort which brings together education, health and wellbeing research in line with the Curriculum for Wales framework for health and wellbeing. Health, education and social care data of primary school children are linked and held in the Secure Anonymised Information Linkage (SAIL) Databank. In addition, school-aged children can take part in the HAPPEN Survey throughout the academic year to inform design and implementation of the Health and Wellbeing curriculum area based on their pupils' needs. There are over 600 schools registered to take part in the HAPPEN Survey. The linkage of health and education records from the HAPPEN national cohort with the HAPPEN Survey responses gives enriched cohort depth and detail which can be used to extrapolate to other schools in Wales. We present the descriptive data available in HAPPEN, and the future expansion plans. Participants: The HAPPEN cohort includes 37,902 primary-aged school children from 2016-July 2023. Of this number, 28,019 can be linked in SAIL with their anonymised linkage field (ALF). In addition, to date (May 2024), HAPPEN Survey data has been captured from over 45,000 children which can in turn be linked to the electronic data. The survey is completed on an ongoing basis and continues to rise by 7000-8000 responses annually. Findings to date: The child cohort is 49% girls, 47% boys (3% prefer not to state their gender and 1% of this data is missing) and 14% are from an ethnic minority background (10% prefer not to state their ethnicity). Initial findings have explored the impact of Covid-19 on wellbeing and play opportunities. As well as a longitudinal exploration of wellbeing throughout the years. Future plans: HAPPEN is an ongoing dynamic cohort of data collection. Access to the cohort is available through SAIL or HDRUK gateway applications. Ongoing research includes the evaluation of interventions for primary school children such as natural experiment methods, non-means tested free school meal roll-out in Wales, interventions to improve physical literacy including changes to the built environment and interventions to improve health and wellbeing of primary school children.