In biology, iron plays a number of roles. Iron is widespread in most living organisms and is essential for their function. In microbes, iron redox chemistry is utilized as an electron donor or receptor in microbial metabolism, allowing microbes to generate energy. In the oceans, iron is essential for the growth and survival of phytoplankton, which use iron to fix nitrogen. Iron is also important in plants, given that they need iron to transfer electrons during photosynthesis. Finally, in animals, iron plays many roles, however, its most essential function is to transport oxygen in the bloodstream throughout the body. Thus, iron undergoes many biological processes, each of which have variations in which isotopes of iron they preferentially use. While iron isotopic fractionations are observed in many organisms, they are still not well understood. Improvements in the understanding the iron isotope fractionations observed in biology will enable the development of a more complete knowledge of the enzymatic, metabolic, and other biologic pathways in different organisms. Below, the known iron isotopic variations for different classes of organisms are described.