This review deals with the properties of fermented diets and their effects on growth performance and gastrointestinal environment of pigs. In addition, some possible modes of action are hypothesized. Starch and sugar rich liquid co-products have a high potential for fermenting during storage. Soaking compound feed with water is another means of achieving a fermented diet. These diets are characterized by a pH between 3.5 and 4.5, high levels of lactic acid, and, to a lesser extent, acetic acid and alcohol. Fermented diets seem to improve growth performance of pigs, compared with non-fermented diets. The exact reasons for this are not yet clear; however, some hypotheses are given. Based on a limited number of studies, fermented diets reduce the gastric pH and the number of Coliform bacteria in the gastrointestinal tract compared with non-fermented diets. Furthermore, there are some indications that fermented diets may positively affect pancreatic secretion, villus architecture, digestibility and absorption of dietary nutrients. Fermented diets may reduce the physical activity of pigs. More specific studies on the effect and modes of action of fermented diets are needed to allow firmer conclusions to be drawn.