Membranes have become an integral part of biotechnological processes. They have proved to be useful tools for the retention of biocatalysts in industrial biotransformations. This paper describes the development of membrane reactors and the application of membrane-based processes in biotransformations. Membrane systems for substrate dosing (e.g., aeration), catalyst retention (e.g., ultrafiltration), product separation (e.g., pervaporation), and salt removal (electrodialysis) from solutions are dealt with. The use of membranes in bioelectrochemistry is also discussed.