In the last decade, few substances have been discussed as controversially as vitamin D. In the last few years, vitamin D research has now also found its way into the intensive care unit (ICU). Vitamin D deficiency is commonly found in the ICU and is associated with adverse outcomes including excess mortality, longer length of stay, higher sepsis incidence, longer mechanical ventilation. But how should one single vitamin be capable of such an impact? It has to be kept in mind that vitamin D is not a classic vitamin at all. It can be synthesized in sufficient amounts by the human body, it has a nuclear receptor and a large number of genes are under direct or indirect control of vitamin D. Furthermore, both the vitamin D receptor and the 1-α hydroxylase which is required to activate vitamin D are widely distributed in the human body. Unfortunately, as in other settings, a large body of observational data is opposed to only a few intervention studies. This article seeks to review the current observational and interventional literature concerning vitamin D status in the context of critical care, its effects on this highly vulnerable population and possible treatment strategies as well as an outlook on research that is necessary in the future.