The microbiome, of which the bacterial component alone (microbiota), is estimated to include 10 times more cells than human cells of the body, blooms immediately after birth and evolves in composition and complexity throughout childhood. The gut microbiome has a profound impact on gastrointestinal tract development, maintenance of mucosal surface integrity, and contributes to the nutritional status of the host and thus plays a pivotal role in health and disease. New technologies have enabled the detailed characterization of normal microbial symbionts and dysbiosis-disease associations. This review summarizes the stepwise establishment of the intestinal microbiota, influential environmental factors, and how this may be perturbed in preterm very-low-birth-weight infants. The contribution of the microbiota to provision of energy and nutrients for intestinal development and the nutritional status of the host are reviewed. In addition, the crucial role of the gut microbiota in maintaining mucosal integrity is explored along with how its breakdown can lead to sepsis, necrotizing enterocolitis, and systemic inflammatory response syndrome. Finally, the role of enteral feeding type (human milk, formula, and nutrient fortification) in mediating these processes is discussed, and guidance is provided for nutritional strategies to promote health in these fragile infants.