Since ancient times, it has been believed that lactic acid bacteria have a beneficial effect on humans. It is known that Pliny the Elder recommended fermented drinks from milk to treat gastrointestinal ailments. However, as late as in the 20^th century, Ilia Miecznikow, a Russian scientist and Nobel Prize winner was the first to provide a scientific basis for this statement. He linked the human health and the presence of a specific group of micro-organisms in gastrointestinal tract; in particular, his interest was devoted to bacteria contained in kefir or yoghurt eaten by people. Since that time, scientific investigations have been conducted in order to study and to characterize the intestinal micro-flora and the function of individual bacterial strains present therein. Additionally, searches have been continued with the aim to find appropriate strains of milk bacteria to exert advantageous impact on human health. In this paper, the history and definitions of probiotics are presented, as well as their beneficial effects on the functions of human organism and on the disease risk reduction. The mechanisms of probiotic effects are characterized as are the commercial products containing probiotic strains and their dosing.