This chapter describes some of the key features of processes in animals and microorganisms before moving on to consider recent work with the iron storage protein ferritin and its bacterial equivalent, bacterioferritin (bacfer). A general review of iron metabolism is also provided a background to the subject of aluminum biochemistry. Aluminum—a toxic element with known beneficial role in animals—appears to be taken in and transported in animals in a fashion similar to their uptake and transport of iron. This topic is also considered in the present chapter, with particular emphasis on the nature of aluminum species present in blood plasma. Mineral nutrition of plants is a complex topic with, in many cases, iron being a limiting growth factor and aluminum being harmful. Some aluminum-tolerant species are known—such as tea plants—that are actually aluminum accumulators and are given alum as a fertilizer. In these plants, the aluminum is stored in the leaves, a fact that has given to rise to some concern about human ingestion of aluminum, though much of this concern seems misplaced. © 1991 Academic Press, Inc.