Vitamin D deficiency and insufficiency is common in children. Vitamin D deficiency is defined as a 25-hydroxyvitamin D <20 ng/mL and insufficiency as 21-29 ng/mL. The major sources of vitamin D are sun exposure, diet, and supplements. In young children, vitamin D deficiency causes a mineralization defect of the skeleton, resulting in rickets. Vitamin D deficiency has been linked to many chronic diseases, including multiple sclerosis, type 1 diabetes, infectious diseases, heart disease, and type 2 diabetes. In utero vitamin D deficiency increases risk for preeclampsia and the need for a cesarean section. Children living at higher latitudes and who are presumed to be at increased risk for vitamin D deficiency are at higher risk for developing type 1 diabetes and multiple sclerosis later in life. The Institute of Medicine recommends that children 0-1 year and 1-18 years need 400 and 600 IU/d, respectively, of vitamin D for maximal skeletal health. The Endocrine Society's Clinical Guidelines recommended that to prevent and treat vitamin D deficiency, children 0-1 year and 1-18 years need 400-1000 and 600-1000 IU/d, respectively.