Fermented (also "cultured") dairy products are among the most traditional foods, included in the cuisines of almost all major countries. Fermentation is one of the oldest food preservation methods. While the basic aspects of all dairy fermentations as a preservation method are based on the partial conversion of one of the main milk constituents (the lactose) into lactic acid, the flavourful by-products of the various lactose conversion pathways result in great variety of traditional cultured dairy foods. The production of these various flavour compounds, in turn, depends on the use of specific microbial cultures. More recently, a new category of fermented dairy products has emerged, combining the traditional functions of dairy cultures with proven or alleged health-promoting properties of several specific bacterial species. In a still wider application, general fermentation technology is now being intensively studied with a specific dairy focus for production of compounds that can be used as natural food preservatives, natural nutraceutical products such as bioactive peptides, or as natural flavour ingredients. Traditional or novel dairy organisms are being considered as sources of specific enzymes and/or enzyme cocktails that can be produced by disruption of the microbial cultures by various mechanical or other means and used with minimal or no further purification. In this application, the fermentation technology offers almost unlimited opportunities for development of new dairy foods and ingredients from various traditional or novel dairy materials.