The article reviews the progress of investigations of b-galactosidases from microbial sources. These enzymes show great differences in optimal conditions of lactose hydrolysis and their utilisation create new possibilities to improve milk and milk by-products processing. Some beta-galactosidases from extreme thermophiles have significant activity above 100?C. Possible applications and interrelationship of both molecular structure and thermostability of these enzymes are also discussed.