This document presents a unique and exhaustive review of the state-of-knowledge on the use of probiotics in diverse livestock production systems, and their impact on animal productivity. It focuses speciﬁcally on deﬁnitions, production, mechanisms of action, applications, effects, safety and potential public health risks of probiotics. In addition the labelling of probiotic products and global regulatory status of probiotics in animal feed is also covered. This publication will inform those that are interested in identifying and designing interventions for increasing animal productivity. It would also give an impetus to the development of new probiotics having consistent long-term effects that could possibly be used in feed in place of antibiotic growth promoters.