Abstract Whey (milk serum) is produced by the dairy industry during the manufacture of cheese. In addition to being a valuable source of functional and nutritional proteins, whey also presents almost all the lactose from the original whole milk. However, many industries still consider the whey as an effluent, which can cause serious environmental problems when not properly treated. Therefore, it is important to develop alternatives for the adequate use of whey. The lactose obtained from whey permeate can be converted into lactulose, a prebiotic which may be metabolized in the intestine by probiotic bacteria, such as Lactobacillus sp. and Bifidobacterium sp., through enzymatic isomerization or by using alkaline catalysts, with minimal secondary reactions and high yield. This manuscript provides information about various techniques used to produce lactulose, its purification and analysis, as well as its mechanisms of action and alternative applications in food products and medicines.