Published by Nowland, Kirkwood and Pluske in Animal The international journal of animal biosciences, this review looks at the existing literature in pigs and other species which demonstrates that a major determinant of the health and performance of animals is microbial colonisation of the GIT in early life. Additionally, the mother, pen environment and general husbandry practices such as cross-fostering and antibiotic administration to sows and piglets also influence the intestinal microbiota, therefore also impacting long-term piglet health, performance and survival.