The role of vitamin D in calcium and phosphate homeostasis is well known; however, in addition to traditional functions, vitamin D modulates a variety of processes, and evidence shows that it has an important role in different allergic diseases such as asthma, atopic dermatitis, and food allergy. Vitamin D acts by binding to the vitamin D receptor, which is present in a variety of tissues; for this reason it is considered a hormone. One of the most important functions is to modulate the immune system response, both innate and adaptive, by suppressing Th2-type response and increasing natural killer cells. Recent studies show that higher serum levels of 25-hydroxyvitamin D were associated with a reduced risk for asthma exacerbations and hospitalization. Other experimental data suggest that vitamin D can potentially increase the therapeutic response to glucocorticoid and potentially be used as an add-on treatment in steroid-resistant asthmatic patients. However, vitamin D stimulates the production and regulation of skin antimicrobial peptides, such as cathelicidins, which have both direct antimicrobial activity and induced host cellular response by triggering cytokine release. Recent evidence suggests that low blood vitamin D level is a risk factor for food allergy; vitamin D deficiency predisposes to gastrointestinal infections, which may promote the development of food allergy. In conclusion, several data suggest that serum 25-hydroxyvitamin D levels are often insufficient in children with asthma, atopic dermatitis, and food allergy. Further clinical trials are needed to provide conclusive evidence and to identify the effects of vitamin D in allergic diseases.