Humans and animals are colonized by multituded of commensal organisms that are essential for growth and development of hosts. Studies on host-bacterial symbiotic relationship could conducive to improve the health status of humans or animals and may contribute to clinical diagnosis and prevention of diseases. In this article we review the Seven aspects detailed in types of symbiotic relationship, postnatal establishment of gut microflora, microbial diversity, adhesion of microbes to gut, bacterial-epithelial crosstalk and their effects on intestinal development anf energy balance, mucosal immunity and disease treatment.［Chinese Journal of Animal Nutrition, 2010，22（3）:519-526］