Improving wellbeing among adolescents and young adults aged 15 to 34 years forms an integral part of universal health coverage and sustainable development in sub- Saharan Africa (SSA). However, data on the wellbeing effects of policies and programmes for this age-group remain scarce, despite this being a key Sustainable Development Goal. The lack of evidence on wellbeing determinants and measures are key barriers to the design and evaluation of wellbeing policies and programmes. This thesis contributes towards informing wellbeing measures for policy evaluations and identifying policies and programmes that may be effective in promoting wellbeing among this population. In this thesis I aim to investigate appropriate measures and health determinants of wellbeing among adolescents and young adults in SSA. Using a mixed-methods research design, I first identify the correlates and experiences of wellbeing among young people living with HIV via a mixed-method review. Thereafter, I apply econometric techniques to investigate the relationship between health (mental and physical) and wellbeing; and subsequently examine whether scale-up of HIV treatment, a key public health policy in this setting, has been associated with any wellbeing gains among young people as they transition into adulthood. Lastly, I qualitatively explore the local understandings and experiences of wellbeing among young people living with and without HIV. In my review, I find that social support and belonging were key correlates of wellbeing, and that acceptance and belonging within networks were key in shaping experiences suggestive of wellbeing. In my econometrics study, I find that poor mental health and physical health are negatively associated with wellbeing, and that HIV treatment access is strongly positively correlated with wellbeing. My qualitative study suggests that social integration and social contribution are key dimensions of young people’s wellbeing in this setting. Overall, my thesis supports the use of multi-dimensional relational wellbeing measures for this population. Evaluation of multi-sectoral HIV policies on wellbeing among adolescents and young adults are warranted.