Milk is a biological fluid a very complex structure, yellowishwhite in color, characteristic taste and smell, which is excreted from mammary glands of women, a certain time after the birth. Milk is the first food for young mammals, including infants, but also a very important food during the entire lifespan of human. It contains energetically important substances, but also certain protective substances that are essential for normal growth and development of the young organism, preschool and school-age children, adolescents, and adults. Exept cow's milk, there is sheep's, goat's, buffalo's and many other different kinds of milk. Milk is a food that can be processed into many dairy products, such as various types of cheese, cream, butter, fermented milk and others. Fermentation is a process that leads to biochemical changes in organic constituents of action of microorganisms, that conduct series of oxidation-reduction reactions, usually without oxygen, with the release of energy. The most important microbial cultures for the production of fermented dairy products are: mesophilic culture LAB2, thermophilic cultures LAB, probiotic LAB and mixed cultures of LAB and yeasts. Just like milk, fermented dairy products have high nutritional value, too. In late 19th and early 20th century, the most populated fermented product was yogurt, which then has conquered the Balkan markets at the time. That was contributed by the research and work of the famous Russian scientist Ilya Mečnikov, who assumed that the Bulgarians are the long – lived because of the long – term consumption of yogurt. Except the yogurt, very important type of fermented milk is buttermilk. Definition of buttermilk is very flexible, depending on the country where is has been producted. Discoveries of the presence of certain microorganisms that can colonize the human intestinal tract have helped in maintaining the health of infants and children with gastrointestinal infections. Lactic acid bacteria, yeasts and molds are used in a wide range of fermented dairy products, but only some of them have probiotic properties. Fermented dairy products have a significant impact on human health. They have beneficial effect on the gastrointestinal tract, products containing bifidobacteria can correct disorder of intestinal microflora , they prevent traveler's diarrhea and others. Probiotics or "beneficial" bacteria are live microorganisms which, when used in humans, have beneficial effect to the host by changing the properties of microflora of the digestive system and internal ecosystem. We can conclude that consumption of milk and fermented dairy products is very important. They have a beneficial effect on human health from our first days of life, and from first days of our life throughout the whole period of the human existence.