A lot of people are vitamin D deficient, what with our 21st century habit of working indoors and not spending enough time outdoors, especially in the winter months. I make sure to take this in the winter when I'm not really in the sun in order to keep my vit D levels up. Not only that, but since I have a history of depression I take this even in the summer, because having enough vit D helps with depression. An all natural happy pill, so to speak, when sunlight isn't available!