FeII/alpha-ketoglutarate (alphaKG)-dependent hydroxylases catalyze an amazing diversity of reactions that result in protein side-chain modifications, repair of alkylated DNA/RNA, biosynthesis of antibiotics and plant products, metabolism related to lipids, and biodegradation of a variety of compounds. These enzymes possess a beta-strand "jellyroll" structural fold that contains three metal-binding ligands found in a His1-X-Asp/Glu-Xn-His2 motif. The cosubstrate, alphaKG, chelates FeII using its C-2 keto group (binding opposite the Asp/Glu residue) and C-1 carboxylate (coordinating opposite either His1 or His2). Oxidative decomposition of alphaKG forms CO2 plus succinate and leads to the generation of an FeIV-oxo or other activated oxygen species that hydroxylate the primary substrate. The reactive oxygen species displays alternate reactivity in related enzymes that catalyze desaturations, ring expansions, or ring closures. Other enzymes resemble the FeII/alphaKG-dependent hydroxylases in terms of protein structure or chemical mechanism but do not utilize alphaKG as a substrate. This review describes the reactions catalyzed by this superfamily of enzymes, highlights key active site features revealed by structural studies, and summarizes results from spectroscopic and other approaches that provide insights into the chemical mechanisms.