In the UK, as in virtually every developed country, it is widely accepted that we need more people studying and working at all levels in Science, Technology, Engineering and Mathematics (STEM). STEM industries are vital elements of the global economy with jobs in science, technology and engineering predicted to grow at double the rate relative to other careers. Yet there is a widespread consensus that there is a substantial - and growing - STEM skills gap, with insufficient numbers of suitably STEM-qualified workers to meet demand. It has also been argued that STEM skills are beneficial for a wide range of careers and can promote social mobility. Relatedly, there are serious concerns about the lack of STEM-qualified graduates entering teaching and the potential impact of this shortfall not just on schools currently, but also for the future STEM skills gap. Alongside the need to increase STEM participation, important arguments have been made for the imperative to widen participation in STEM and ensure high levels of scientific, mathematical, technical and digital literacy across the population. Specifically, there is a need to broaden the gender, ethnic and social class profile of those who study STEM post-16, particularly in the physical sciences and engineering, where women, some minority ethnic and working-class communities are starkly under-represented. Yet initiatives aimed at increasing and/or widening the profile of STEM graduates appear to have had little lasting impact on the higher education participation rates. Understanding the factors shaping STEM participation is, therefore, a key priority area for governments and a wide range of stakeholders both nationally and internationally. The proposed three year study seeks to understand the processes through which young people develop their science and career choices and trajectories from age 20-23. Specifically, the proposed study will extend the unique dataset developed by the first and second longitudinal ASPIRES and ASPIRES2 studies, which tracked the development of young people's science and career aspirations from age 10-19 (surveying over 39,000 young people at five time points between the ages of 10 and 18 and longitudinal, repeat interviews with 61 young people and their parents over the same age period). ASPIRES3 will continue tracking this cohort via a representative national survey with c.7-10,000 young people at age 20/21, sampled from those who have previously conducted ASPIRES/ASPIRES2 surveys and boosted via online (social media) recruitment, as a media that is particularly successful for recruiting this age group. The project will also conduct interviews with c.60 students who have been longitudinally tracked from age 10 and their c.60 parents. The project will also undertake secondary analysis of previous ASPIRES & ASPIRES2 survey data from over 16,000 students, matched to large national data bases to examine whether/which attitudinal and social factors at age 10/11 relate to later attainment and life outcomes at age 20/21. The knowledge generated by the research will inform inter/national STEM education policy and practice, particularly how to better increase and widen post-compulsory participation in Science, Technology, Engineering and Mathematics. The project will become the only longitudinal project to track young people's aspirations (in and out of science) from primary school, through compulsory, post-compulsory and higher education into work. In line with our commitment to achieving impactful research, the study will involve three impact collaborations with the Royal Society of Chemistry, Engineering UK and the Institute of Physics. Findings and recommendations will be disseminated via a wide range of academic journal articles and bespoke summaries, publications, events and social media for stakeholders.