Individuals are capable of producing vitamin D with proper exposure to sunlight. However, several factors can interfere with the effectiveness of this process. Most sunscreens filter out UVB light, thus inhibiting vitamin D production. Individuals with more darkly pigmented skin have greater difficulty producing vitamin D because melanin acts as an effective natural sunscreen, requiring longer sun exposure to produce an adequate daily allotment of vitamin D. Additionally, solely breastfed infants whose mothers suffered from vitamin D deficiency or insufficiency when pregnant have smaller reserves of the nutrient and are at greater risk of developing nutritional rickets. Vitamin D deficiency leads to rickets, osteomalacia, and osteoporosis. Long-term vitamin D insufficiency can lead to paracrine effects such as type 1 diabetes, cancer, and multiple sclerosis. This article reviews the current literature on vitamin D deficiency and insufficiency and their relation to different disease states. Potential areas for research are discussed.