Exogenous enzymes are used in pig diets to improve the availability and digestibility of some non-accessible nutrients. As result of this enhanced digestion, short fragments of these molecules may become available in the distal foregut and the hindgut and modulate microbiota composition, gut barrier integrity, and overall animal health. This chapter reviews the effects of different exogenous enzymes (carbohydrases, phytases, proteases and lipases) on nutrient digestibility, gut microbial ecology, and barrier function and immunity of pigs at different ages (sows, weaned piglets, growing/fattening pigs). Exogenous enzymes are usually included into feeds as blends so they can complement each other’s activities and further improve the accessibility to non-digestible structures. Exogenous enzymes used in feed manufacturing for more than 30 years, initially to improve the digestive function of non-digestible nutrients (i.e. fibre, phytic acid, etc.), more recently other indirect actions on the regulation of gut microbiota and gut health have gained interest.