Vitamin D deficiency is a common occurrence globally and in particular, in at risk populations such as young children, pregnant women and the elderly. This is mainly due to the lack of natural dietary sources of vitamin D and the limited cutaneous production of vitamin D3. Vitamin D has been associated with a number of non-skeletal diseases and outcomes e.g. cardiovascular disease (CVD), diabetes, pre-eclampsia and low birth weight (Muscogiuri et al. 2012; Beveridge & Witham 2013; Pérez-López et al. 2015), however, these associations remain to be proven. The aim of this thesis was, firstly, to examine the incidence of vitamin D deficiency in a number of cohorts across the lifecycle, using a validated Ultra-Performance Liquid Chromatography- Tandem Mass Spectrometry (UPLC-MS/MS) assay. A second aim was to examine the associations between 25OHD and a number of non-skeletal health outcomes across the lifecycle, and, thirdly and finally, to determine the dietary and supplement sources of vitamin D in two of these cohorts.The measurement of vitamin D metabolites was conducted using a validated UPLC-MS/MS assay. Vitamin D deficiency (<25 nmol/L) was evident in all three cohorts, 26.6%, 40.0% and 14.8%, in the Hyperglycemia and Pregnancy Outcomes study (HAPO), the MEDiterrean Diet in Northern Ireland (MEDDINI) trial and the EURopean EYE (EUREYE) study, respectively. 25-hydroxy vitamin D (25OHD) was not associated with glucose, insulin or lipid metabolism in pregnant women. A positive association was found with maternal 25OHD and birth weight SDS, in addition, a risk reduction for low birth weight as maternal 25OHD increased was observed. An association was also observed between maternal 25OHD and a marker of cord beta cell function. In the older European cohort, an association, following adjustment for a number of confounders, was detected between 25OHD and diabetes, with no association observed between 25OHD and CVD in the same cohort.It was observed that a Mediterranean diet (MD) significantly increased 25OHD levels in a Belfast CVD patient group and this was more than likely due to increased fish intake. This was further reiterated in the EUREYE cohort where oily fish was significantly associated with increased 25OHD levels. Furthermore, in the EUREYE cohort, despite the lack of a seasonal effect on 25OHD levels, Bergen, had the highest concentration of 25OHD. This finding in Bergen, may be attributed to fish oil/omega-3 supplements as it was the only dietary source associated with 25OHD levels.Vitamin D deficiency was found to be commonplace across the lifecycle, and across Europe. The current analysis suggests that further investigation may be warranted on the relationship that exists between maternal 25OHD levels and the possible health implications for the offspring, such as risk of chronic diseases later in life. The findings also suggest that the use the MD diet can increase dietary vitamin D intake. This is more than likely due to the increased consumption of oily fish, which is supported by the association found in the EUREYE cohort. Furthermore, findings from the present analysis suggest that it may be possible to increase 25OHD levels to a sufficient level from oily fish or fish oil supplements only, as demonstrated by the findings from the Bergen centre in the EUREYE study.In conclusion, vitamin D deficiency is a mutual problem shared across the lifecycle. Practical and achievable methods to eradicate vitamin D deficiency should be established by public health agencies. There is a possibility that vitamin D deficiency may be associated with chronic diseases and other health outcomes throughout the lifecycle. Minimising vitamin D deficiency may help to reduce the risk for these health outcomes, however, causality still remains to be proven.