Enzymatic hydrolysis of lactose is accompanied by transgalactosylation reaction, thereby producing oligosaccharides. The prebiotic functions of oligosaccharides such as bifidogenic effect and health benefits connected with this effect were described. The mechanism of galactooligosaccharides formation and factors affecting it were also presented. Moreover, the examples of an enzymatic enrichement of milk and it's derivatives in galactooligosaccharides were given.