Obesity, which prevalence is increasing worldwide, is often characterized by low blood vitamin D level, which may relate to obesity-associated health risks like reduced insulin sensitivity. This dissertation aims to investigate the association between vitamin D and insulin sensitivity in individuals with obesity and insulin resistance. The studies presented in this thesis suggest that low vitamin D concentrations were not associated with insulin resistance in obesity. Furthermore, this research found that vitamin D supplementation did not improve tissue-specific insulin sensitivity in individuals with or at risk of insulin resistance.