The human intestinal microbiota is composed of 10 to 100 trillion microbes whose metabolic activity equals to a virtual organ within an organ. Gut microflora have a crucial role in the maintenance of intestinal homeostasis. The composition of gut microflora is changing along the gastrointestinal tract. Factors that affect colonization of newborn's gut microbiota are delivery mode, type of feeding, illness and prematurity Our diet has a dominant role in shaping the microbial composition of the gut over other inviro-mental factors. The aim of this article is to introduce up-to-date knowledge of the organization and function of gut microflora.