OBJECTIVES: This paper reviews the published evidence on early-life intestinal microbiota development, as well as the different factors influencing its development before, at, and afterbirth. A literature search was done using PubMed, Cochrane and EMBASE databases. A growing body of evidence indicates that the intrauterine environment is not sterile as once presumed, but that maternal-fetal transmission of microbiota occurs during pregnancy. The consecutive order of bacteria with which the gastrointestinal tract is colonized will influence the outcome of community assembly and the ecological success of individual colonizers. The genetic background of the infant may also strongly influence microbial colonization of the gastrointestinal tract. The composition and development of infant gut microbiota can be influenced by many prenatal factors, such as maternal diet, obesity, smoking status, and use of antibiotic agents during pregnancy. Mode of delivery is generally accepted as a major factor determining the initial colonization. Breast milk stimulates the most balanced microbiome development for the infant, mainly because of its high content of unique oligosaccharides. Feeding is another important factor to determine intestinal colonization. Compared with breastfed infants, formula-fed infants have an increased richness of species. Initial clinical studies show that infant formulas supplemented with specific human milk oligosaccharides (HMOs) -2´-fucosyllactose alone or in combination with lacto-n-neotetraose are structurally identical to those in breast milk. HMOs increase the proportion of infants with a high bifidobacterial-dominated gut microbiota typical of that observed in breastfed infants, lead to plasma immune marker profiles similar to those of breast-fed infants and to lower morbidity and antibiotics use. Further clinical studies with the same, others or more HMOs are needed to confirm these clinical effects. A growing number of studies have reported on how the composition and development of the microbiota during early life will affect risk factors related to health up to and during adulthood. If exclusive breastfeeding is not possible, the composition of infant formula should be adapted to stimulate the development of a bifidobacterial-dominated gut microbiota typical of that observed in breastfed infants. The main components in breast milk that stimulate the growth of specific bifidobacteria are HMOs.