The ferritins of animals and plants and the bacterioferritins (BFRs) have a common iron-storage function in spite of differences in cytological location and biosynthetic regulation. The plant ferritins and BFRs are more similar to the H chains of mammals than to mammalian L chains, with respect to primary structure and conservation of ferroxidase center residues. Hence they probably arose from a common H-type ancestor. The recent discovery in E. coli of a second type of iron-storage protein (FTN) resembling ferritin H chains raises the question of what the relative roles of these two proteins are in this organism. Mammalian L ferritins lack ferroxidase centers and form a distinct group. Comparison of the three-dimensional structures of mammalian and invertebrate ferritins, as well as computer modeling of plant ferritins and of BFR, indicate a well conserved molecular framework. The characterisation of numerous ferritin homopolymer variants has allowed the identification of some of the residues involved in iron uptake and an investigation of some of the functional differences between mammalian H and L chains.