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In Wales nearly 5% (160,000) of the population have been diagnosed with diabetes, and it is thought that there is another 66,000 undiagnosed cases (Diabetes UK). Vitamin D deficiency is also increasing in prevalence, and there is emerging evidence linking vitamin D deficiency to impaired β-cell function, insulin resistance, and glucose intolerance, all of which are central to the pathogenesis of type 2 diabetes (Song & Manson, 2010). Following NHS and University ethical approval Welsh domiciled participants (n = 116) with varying levels of glucose control attended the Department of Sport and Exercise laboratory three times over a twelve-month period to examine the relationship between vitamin D and glucose homeostasis. Significant associations were observed between 25(OH)D and measures of glycaemia, and a seasonal variation was observed in 25 hydroxyvitamin D (25(OH)D) in this Welsh population (summer 71.3 ± 23.8; winter 42.6 ± 23.8 nmol/l). Participants with normal glucose control (≤ 6 mmol/l) had significantly (U = 884.00, p = 0.03) higher 25(OH)D concentrations than those with abnormal glucose control (≥ 6.1 mmol/l). A randomised control trial failed to find an effect of vitamin D supplementation (2000 IU/day) and a fifteen-week cycling programme on measures of glycaemia and body composition in 36 healthy participants. There was a significant (23%) increase in vitamin D status in participants in the supplementation groups demonstrating the effectiveness of the dose administered. Combined with the body of evidence in this area (Pittas et al., 2007) the findings from this thesis provide some support for the potential role of vitamin D supplementation in the management and prevention of type 2 diabetes.