Vitamin D is synthesised endogenously within the epidermal layer of the skin on exposure to ultraviolet B radiation (UVB). Therefore, inadequate exposure to UVB results in vitamin D insufficiency, for example during winter at high latitudes when the availability of solar UVB is negligible, or due to living indoors for the majority of sunlight hours and heeding recommendations to cover-up from the sun. Avoiding low vitamin D status (serum 25-hydroxyvitamin D (25(OH)D)) is accepted as important for bone health because the primary physiological function of vitamin D is to maintain calcium and phosphorus homeostasis. The identification of the vitamin D receptor in various tissues (including the cardiovascular system and skeletal muscle), has led to recent evidence suggesting vitamin D may also affect exercise performance. This thesis set out to investigate the potential role of vitamin D in optimising exercise performance among young healthy adults. First, in a prospective cohort study (n = 967; Chapter 3), it was demonstrated that vitamin D insufficiency was prevalent among male and female military recruits during winter (91% of males and 64% of females: 25(OH)D <50 nmol·L-1), and vitamin D status was positively associated with endurance exercise performance (P < 0.01, ΔR2 = 0.03–0.06; 1.5-mile run time was ~half-a-second faster for every 1 nmol·L-1 increase in 25(OH)D). No significant effects on muscular strength or power emerged (P > 0.05). Next, to restore and then maintain vitamin D sufficiency from its winter nadir (n = 33), closely matched vitamin D supplementation protocols using safe simulated sunlight or oral vitamin D3 were demonstrated to be effective in a 12-week randomised placebo controlled trial (89% at week 5 and 100% at week 12: 25(OH)D ≥50 nmol·L-1; Chapter 4). Finally, in a randomised placebo controlled trial among 137 male military recruits, despite 12-weeks simulated sunlight in accordance with recommendations on safe sunlight exposure and oral vitamin D3 supplementation restoring and then maintaining vitamin D sufficiency in almost all (97%), there was no effect on exercise performance (P > 0.05; Chapter 5). This lack of beneficial effect was observed despite the majority of supplemented individuals achieving 25(OH)D ≥75 nmol·L-1, which has been proposed to be optimal. These findings suggest vitamin D does not directly affect exercise performance.