In order to assess the influence of vitamin D on in vivo immunity, there is a need for well-controlled field studies and randomised placebo-controlled trials to be conducted among young healthy adults, using clinically relevant in vivo measures of immune competence. The broad aim of this thesis was to investigate the influence of vitamin D on the development of in vivo immunity, as assessed by the secondary antibody response to hepatitis B vaccination (anti-HBs). Firstly, in a prospective cohort study of 447 healthy young men and women, we examined the influence of vitamin D status (serum 25(OH)D) on secondary hepatitis vaccination response. We demonstrated that fewer secondary hepatitis B vaccine responders were observed among vitamin D insufficient individuals, and those with the lowest 1,25(OH)2D concentrations. These associations were particularly stronger in men compared to women, likely due to a greater prevalence of vitamin D insufficiency in men compared to women (serum 25(OH)D ≥ 50 nmol/L; men vs. women, 49% vs. 70%). These findings may in part explain the observed seasonal variation in secondary hepatitis B vaccination response, in that fewer men and women were vaccine responders in the winter compared to summer (44% vs. 62%, respectively). This seasonal variation in secondary hepatitis B vaccination response also mirrored seasonal variation in serum 25(OH)D and 1,25(OH)2D concentrations as well as the higher prevalence of vitamin D insufficiency during the winter (Chapter 4). Then we demonstrated that simulated sunlight and oral vitamin D3 supplementation restored 25(OH)D from winter concentrations to concentrations typical during summer within 4 weeks and maintained IOM and EFSA vitamin D sufficiency (serum 25(OH)D ≥ 50 nmol/L) concentrations for a further 8 weeks (Chapter 5). After demonstrating that the change in serum 25(OH)D was matched between simulated sunlight and oral vitamin D3 (Chapter 5), we applied these methods to a randomised placebo-controlled trial that investigated the effect of 12-weeks vitamin D supplementation on the secondary hepatitis vaccination response in 119 healthy young men. Results showed that vitamin D supplementation, via simulated sunlight and oral vitamin D3 supplementation did not influence secondary anti-HBs response (Chapter 6). Future studies should seek to raise vitamin D status prior to initial vaccination; using methods that replicate safe, practical and government recommended summer sunlight and oral vitamin D3 supplementation guidelines.