Corrinoids are a group of compounds built around the corrin nucleus, a macrocyclic ring of four reduced pyrrole rings, and they include vitamin B12, the most famous member. Many corrinoids carry a central cobalt atom, which is essential to vitamin B12’s biological activity in cell division, DNA synthesis, and neurological function. Vitamin B12 occurs naturally in animal-derived foods but not in plant foods, although certain bacteria also synthesize corrinoids. This distribution makes B12 status a key concern in vegetarian and vegan nutrition, addressed through fortified foods and supplements with verified bioavailability.