Liquid feeding involves the use of a diet prepared either from a mixture of liquid food industry by-products and conventional dry materials, or from dry raw materials mixed with water. By definition fermented liquid feed is feed that has been mixed with water, at a ratio ranging from 1:1.5 to 1:4, for a period long enough to reach steady state conditions. If there is almost no time between mixing and feeding or the period for fermentation is too short to reach steady state conditions, the term liquid feed or non-fermented liquid feed is used [1].By mixing with water, lactic acid bacteria and yeasts naturally occurring in various feed ingredients proliferate and produce lactic acid, acetic acid and ethanol which reduces the pH of the mixture [2]. This reduction in pH inhibits pathogenic organisms from developing in the feed [3]. In addition, when this low pH mixture is fed, it reduces the pH in the stomach of pigs and prevents the proliferation of pathogens such as coliforms and Salmonella from developing in the gastrointestinal tract [2].The interest in the fermentation of feed for improving the performance of piglets and pigs increased dramatically after the announcement of the ban in the European Union on the use of antibiotics as antimicrobial growth promoters for swine. The potential of fermented liquid feed, as an alternative to the use of growth promoting antibiotics has been discussed in four recent reviews [1, 2, 4, 5]. In this review, recent information about the use of fermented liquid feed in pigs will be provided.