There is an increasing need to promote digestive efficiency and productivity while maintaining animal health and welfare. Probiotics are live microorganisms which confer a benefit for the host when administered in appropriate quantities. This chapter begins by reviewing critical periods in the ruminant lifecycle as targets for probiotics. It then looks at definitions of probiotics, delivery mechanisms and regulation. The rests of the chapter summarizes and assesses the range of research on the benefits and modes of action of probiotics, starting with their potential in young ruminants. It then considers the role of probiotics in adult ruminants in the following areas: feed efficiency, methane production, pathogen control and supporting the immune system.