In humans, Fe(ii)- and 2-oxoglutarate-dependent (Fe/2OG) oxygenases are generally of the dioxygenase subclass and mediate hydroxylation of unactivated aliphatic carbon centres. Plants and microbes also employ Fe/2OG hydroxylases and, through investigations of the microbial enzymes, the mechanism of hydroxylation has been established to proceed via a potent high-spin (S = 2) Fe(iv)–oxo (ferryl) complex, which abstracts a hydrogen atom (H˙) from the substrate. Bacteria have further co-opted this central ferryl intermediate for a remarkable array of divergent reactivities, including olefin epoxidations, aliphatic halogenations, olefin-installing 1,2-dehydrogenations, oxacycle-installing 1,3- and 1,5-dehydrogenations, and a redox-neutral stereoinversion. An understanding of the mechanisms leading to this manifold of transformations, and the means by which the individual enzymes direct them, has potential to guide the design of new chemical catalysts and the development of novel bacterially- or chemo-enzymatically-derived drug compounds. In this chapter, we first summarize our understanding of hydroxylation reactions mediated by Fe/2OG hydroxylases and then review recent advances in the elucidation of two of the ‘alternative’ reactivities (halogenation and stereoinversion). Finally, we discuss the remaining, less well understood dehydrogenation reactions, highlighting possible problems with published mechanistic proposals, presenting alternatives to these published mechanisms, and briefly outlining experiments by which the operant mechanisms might be established.