Probiotics are alive micro-organisms, generally bacteria but also yeasts than, when ingested alive in sufficient amount, they have a positive effect on the health going beyond the nutritional ones commonly known. Probiotics may operate through a nutritional and/or health or sanitary effect. Micro-organisms used in animal feed in the EU are mainly bacterial strains of Gram-positive bacteria belonging to the types Bacillus, Enterococcus, Lactobacillus, Pediococcus, Streptococcus and strains of yeast belonging to the Saccharomyces cerevisiae species and kluyveromyces. While most of the species and genera are apparently safe, certain micro-organisms may be problematic, particularly the enterococci, which may harbour transmissible antibiotic resistance determinants and bacilli, specially those belonging to the Bacillus cereus group that are known to produce enterotoxins and an emetic toxin. The history and the current legislation in the European Union on probiotics feed additives including the requirements for the safety assessment for the target animal species, consumers, workers, and environment are presented.