Probiotics are established as feed additives in many animal species including horses. The mode of action is not yet clearly defined. New findings indicate microbiological interactions, functional reactions of the gut epithelium and effects on the immune function. In horses, there is no relevancy of improved production traits. The main indication for using probiotics in this species is health stabilization, especially related to the gastrointestinal microflora, the support of digestive processes and other systemic influences for instance in relation to the immune function. Therefore probiotics are mainly used in patients with gastrointestinal disorders.