In animal nutrition, probiotics are viable microorganisms used as a feed supplement, which lead to beneficial effects for the host animal. For most species a trend towards improved performance has been reported due to the use of probiotics, but statistically significant improvements of weight gain and of feed conversion are rare, mainly because of variations in the individual reactions of the animals. However, reductions of the incidence of diarrhoea were significant in most studies with piglets and calves. In contrast to human nutrition, where Lactobacillus species are common probiotic products, in animal nutrition Enterococcus spp., Saccharomyces yeast, and spore-forming Bacillus spp. are most frequently used as probiotic microorganisms. Furthermore, in human nutrition long-term effects for lasting health promotion and longevity are sought, but the aims of probiotic supplementation in modern animal production are fast effects such as improvement of weight gain and feed conversion. Thus, results from probiotic research in the field of human nutrition are not completely valid for animal nutrition. Although 19 probiotic preparations have received temporary approval in the European Union, their modes of action, which lead to beneficial effects, are only partly known. It is very probable that the impact of probiotics on pathogenic and nonpathogenic intestinal bacteria is of prime importance. However, modifications of microstructures and barrier functions of intestinal tissues as well as reactions of the immune system seem to be involved in the overall effect, directly or as a consequence of modified bacterial populations. In order to specifically improve probiotics and their application in defined indications, more research is needed to determine their underlying mechanisms.