Abstract <p>Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are prosthetic groups of many enzymes and can be attached to proteins either covalently or non-covalently. Covalent attachment of FMN to Thr or Ser residues via a phosphate group is catalyzed by the recently discovered enzyme flavin transferase. Among the enzymes containing phosphoester-linked FMN, the most widely represented ones are various microbial 2-enoate reductases catalyzing reduction of unsaturated carboxylic acids (fumaric, acrylic, cinnamic, urocanic, etc.). The review is focused on microbial 2-enoate reductases and discusses their classification by domain organization and intracellular location, structural basis of substrate specificity, catalytic mechanism, and function, as well as the significance and evolutionary origin of the covalent attachment of FMN as a prosthetic group.</p>