The presentation will take a population health and epidemiological approach to examining vitamin D deficiency in Australia. Firstly some clarity on what denotes vitamin D deficiency is required. Two common cut-points of serum 25(OH)D concentration to define “sufficiency” are 50nmol/L and 75 nmol/L, largely based on requirements for bone health – are these appropriate for other diseases? Several studies report that both high and low 25(OH)D levels are associated with increased disease risks – should a modest upper level of “optimal” be set? Evidence from national studies shows that vitamin D deficiency is common in Australia, despite high levels of ultraviolet radiation (UVR). Yet there are known problems with both accuracy and precision in 25(OH)D measurement, lack of understanding of the functional significance of various epimers (and variable detection across assays) and cross-reactivity with inactive vitamin D metabolites. Ensuring an appropriate sampling frame is critical to estimates of population prevalence, and to define who is most at risk. Australia is the “sunny country”, but it has been difficult to define “how much sun is enough” – for different population groups, living in different locations, at different times of the year - alongside concerns about high incidence of skin cancers and other sun-related diseases. Vitamin D deficiency is clearly a risk factor for bone diseases; a broad range of other diseases is also implicated but with varying levels of evidence. Recent work, particularly in relation to immune function, suggests that sun exposure may be important independently of enhanced vitamin D production. Thus, can all of the benefits of sun exposure be achieved through vitamin D supplementation? The unanswered questions challenge evidence-based public health decision-making, in a context of pressure for clear sun exposure guidance and possibly vitamin D supplementation of foods.