Abstract: Diels-Alder cycloaddition is involved in the biosynthesis of many natural products to construct new C — C bonds and ring systems, resulting in the formation of structurally novel, complex and diverse natural products. Although natural enzymes have been evloved in nature to catalyze this reaction and also received great attention from scientists, researches in this field are still hot topics and facing many difficult and challenging issues. In recent years, with the development of technologies in genome sequencing, bioinformatics and molecular biology, the genes and corresponding enzymes related to Diels-Alder reaction in the biosynthetic pathways of many natural product have been identified and some breakthroughs have been made. This review focuses on the biosynthesis of Diels-Alder adducts from microbial sources, especially the genes and enzymatic mechanisms involved in the Diels-Alder reaction.