Vitamin D is essential in bone mineralization and calcium homeostasis, and an increasing body of evidence suggests that vitamin D may be important for maintaining extraskeletal health, including having beneficial effects on cardiometabolic outcomes. Vitamin D deficiency is widespread, but the role of vitamin D in the metabolic syndrome is not fully elucidated. In this review we summarize data from observational studies and randomized controlled trials on the relation between vitamin D and the metabolic syndrome and its components. A large number of observational studies suggest a relationship between low levels of 25(OH)D and the metabolic syndrome or its individual clinical features. Randomized controlled trials of vitamin D supplementation addressing aspects of the metabolic syndrome have yielded inconsistent results, and many studies suffer from methodological limitations. There is an urgent need for large, well-designed randomized controlled trials with relevant endpoints. Until definitive results from such studies are available, caution should be taken towards the use of vitamin D-supplementation for disorders other than musculoskeletal system. New molecular biological techniques elucidating the interaction between the active vitamin D derivatives and target genes represent a promising approach to more precise knowledge about new biomedical function, which also might shed light on the complex metabolic syndrome.