Vitamin D plays a role in a range of functions that may impact on glycaemic control. In this study we systematically report on clinical studies evaluating the impact of vitamin D on aspects of hyperglycaemia in non-pregnant adults. A total of 1,294 articles, of which 417 were reviews, were identified. No well-designed randomised, controlled trials were identified that specifically investigated the effects of vitamin D supplementation on glucose and insulin concentrations. The majority of the studies that are available were poorly designed, having limited numbers, short study duration, or were conducted in volunteers with normal baseline, as measured by 25-hydroxyvitamin D (25(OH)D), concentrations or used inadequate doses of the supplements to normalise vitamin D concentrations, or used inappropriate analyses. Most studies did not observe improvements in glycaemia, with few exceptions. The results were more equivocal for aspects of insulin resistance. Most found no benefit on measures of insulin resistance, although some did. However, more studies described improved insulin release, although data from the studies to date are really inadequate to provide any reliable conclusions. Well-conducted randomised, controlled trials with adequate vitamin D doses are required to effectively assess whether this vitamin can reduce the incidence of diabetes.