Probiotics are living microorganisms which influence the digestive microflora of the host animal in a beneficial way. They develop their activity exclusively in the digestive tract. In animal nutrition, probiotics used as feed additives belong to one of three different groups: lactic acid bacteria, yeasts and Bacillus spores. They differ from one another in their properties, origin and mode of action. The main activity of probiotics is the maintenance and reconstitution of the equilibrium (eubiosis) of the intestinal microflora which is achieved by various modes of action. The prerequisite for their probiotic action is reaching the gastrointestinal tract alive. Once there, the probiotics support the intestinal microflora by means of specific metabolic activities and/or stimulation of the host's immune system. Undesirable microorganisms are thus reduced and protection is given against colonisation or attachment of harmful microorganisms. Probiotics therefore contribute to averting any disruption of the intestinal microflora (dysbiosis) as may occur during specific growing periods and situations of specific stress for the animals (for instance dietary changes, weaning, regrouping of animals etc.). A safeguard of performance and health is thus achieved. Registration of probiotics follows a uniform EU procedure. Here, probiotics are evaluated especially regarding their quality, efficacy and safety for humans, animals and the environment. Therefore, only welldefined and safe microorganisms are used, for which the bioregulative properties have been validated under conditions of common feeding practice.