Vitamin D is unique, in terms of its metabolism and physiologic features and the human reliance on both endogenous production (activation through exposure to ultraviolet light) and exogenous sources (diet, primarily fortified foods) to meet biological requirements. Recent evidence has indicated a reemergence of vitamin D-deficient rickets and an alarming prevalence of vitamin D insufficiency (ie, low circulating concentrations of 25-hydroxyvitamin D) in particular segments of the US population. Furthermore, evidence has emerged implicating vitamin D status in a range of adverse health conditions, including cancer and certain autoimmune diseases. Therefore, a conference organized by the National Institute of Child Health and Human Development and the National Institutes of Health Office of Dietary Supplements was held to explore current knowledge and to develop a research agenda to address the range of issues associated with vitamin D and health during the life cycle. These proceedings contain presentations about 1) existing data on vitamin D status in the United States and internationally, 2) the current state of knowledge regarding the biological functions of vitamin D, 3) the strength of evidence supporting reconsideration of current policies regarding vitamin D intake, 4) gaps in understanding of the factors affecting and current options for improving vitamin D status in the United States and internationally, and 5) research needs to address gaps in knowledge regarding vitamin D assessment, biological features, and requirements. This executive summary provides an overview of the conference and its conclusions.