This review article describes the occurrence, general properties, and substrate specificity of the flavoenzymes belonging to the glucose-methanol-choline oxidoreductase superfamily and the l-α-hydroxyacid dehydrogenase family. Most of these enzymes catalyze the oxidations of hydroxyl groups, yielding carbonyl moieties. Over the years, carbanion, hydride transfer, and radical mechanisms have been discussed for these enzymes, and the main experimental evidences supporting these mechanisms are presented here. Regardless of the chemical nature of the organic substrate (i.e., activated and non-activated alcohols), a hydride transfer mechanism appears to be the most plausible for the flavoenzymes acting on CH-OH groups. The reaction of most of these enzymes likely starts with proton abstraction from the substrate hydroxyl group by a conserved active site histidine. Among the different approaches carried out to determine the chemical mechanisms with physiological substrates, primary substrate and solvent deuterium kinetic isotope effect studies have provided the most unambiguous evidences. It is expected that the numerous studies reported for these enzymes over the years will be instrumental in devising efficient industrial biocatalysts and drugs.