Along with other organs like prostate, bones and kidney, skin is capable of vitamin D synthesis. Primarily keratinocytes but also macrophages and fibroblasts synthesize active vitamin D from cholesterol precursors by photochemical activation. The synthesized vitamin D functions by binding to nuclear vitamin D receptors. Vitamin D deficiency usually manifests as rickets in childhood although it is today only relevant in diseases characterized by malabsorption due to today's recommended vitamin D prophylaxis. Excessive doses of vitamin D are the usual cause of increased levels. The most common therapeutic target of Vitamin D is psoriasis. Here, topical preparations are usually employed; their anti-proliferative and cell differentiation-promoting action is mediated via binding to cutaneous vitamin D receptors.