Flavoenzymes are omnipresent in nature and are involved in many cellular processes. Flavoenzymes typically contain the vitamin B2 derivatives FAD and FMN as a redox-active prosthetic group. By varying the protein environment around the isoalloxazine ring of the flavin, evolution has created a great diversity of flavoprotein active sites and catalytic machineries. Most flavoenzymes perform one- or two-electron redox reactions and belong to the following groups: Flavoprotein reductases primarily use NAD(P)H as electron donor and pass these electrons to a protein substrate or another electron acceptor. Flavoprotein dehydrogenases oxidize organic substrates and mainly use quinones and electron transfer proteins as electron acceptors. Flavoprotein disulfide oxidoreductases contain active-site thiols. They use a dithiol substrate and NAD+ to form a disulfide product and NADH or act in the other direction yielding NAD(P)+ and a reduced (dithiol) product. Some flavoprotein (di)thiol oxidoreductases stabilize a reactive and reversibly oxidized cysteine in their active site. Flavoprotein oxidases catalyze the conversion of a substrate single bond to a double bond. The reduced flavin generated during this reaction is reoxidized by molecular oxygen to form hydrogen peroxide. Flavoprotein monooxygenases mainly use NAD(P)H as an electron donor and insert one atom of molecular oxygen into their substrates. By doing so, they act in different biological processes, ranging from lignin degradation and detoxification to the biosynthesis of polyketides and plant hormones.