One of the most important discoveries in industrial microbiology is using microorganisms to perform specific chemical reactions. The use of microorganisms for this purpose is called biotransformation. Due to its significant registry and stereoselectivity, biotransformation is an essential tool in the structural modification of organic compounds, especially natural products. Filamentous fungi are frequently used to catalyze selective hydroxylation and side-chain degradation reactions, which are usually difficult to achieve by chemical means. In this book, the hydroxylation and side-chain degradation of progesterone by fungi into several important corticosteroids were discussed.