Saponins are glycosides found in many plants, which have soap character due to their surfactant properties. They work haemolytic and may cause symptoms of intoxication in high concentration. Among therapeutically relevant effects expectorative, anti-inflammatory and immune-stimulating effects have the highest ranking. Beyond that, saponins demonstrate antimicrobial properties particularly against fungi and additionally against bacteria and protozoa. In animal nutrition additions of saponins can suppress intestinal and ruminal ammonia production. The ammonia-reducing effect is primarily attributed to an inhibition of proteolytic microorganisms. Due to reduced NH3-concentrations metabolism is relieved, which offers benefits to animal welfare and animal performance. Accordingly improved feed convertion ratio can be registered in weanling and growing-finishing pigs with the employment of saponin-containing feed additives. With regard to ruminal fermentation in vitro and in vivo studies have shown that beside proteolytic protozoa also cellolytic and amylolytic bacterial species are restrained. To what extent antimicrobial activity supress digestion or whether the selective effects on microorganisms can be used purposefully to manipulate ruminal fermentation, must be clarified in further investigations.