The collection of bacteria, archaea, and eukarya colonizingthe gastrointestinal tract is termed the “gut microbiota.”1) The microbiota offers many benefits to the host, through a range of physiological functions such as strengthening gut integrity or shaping the intestinal epithelium,2) harvesting energy,3) protecting against pathogens,4) and regulating host immunity.5) The newborn infant microbiota is highly dynamic and undergoes rapid changes in composition through the first years of life toward a stable adult-like structure with distinct microbial communities of unique composition and functions at specific body sites.6-9) Markedly, multiple mother-infant studies have indicated the vertical transmission of microbes from mother to infant that can contribute to microbiota colonization.10-11) During early life, the gut microbial composition rapidly changes by maternal microbiota composition, delivery mode, infant feeding mode, antibiotic usage, and various environmental factors, such as the presence of pets and siblings.12) Disruption in the gut microbiota (ie, gut dysbiosis) has been linked to necrotizing enterocolitis in infancy as well as some chronic diseases in later life, including obesity, diabetes, inflammatory bowel disease, cancer, allergies, asthma,13) and neurological diseases associated with the gut-brain axis.14) This review focuses on the process of early colonization to elucidatethe factors influencing microbial colonization of the infant gut.