The human bowel is host to a diverse group of bacteria with over 500 different bacterial species contributing to this diversity. Until recently these bacteria were regarded as residents without any specific functions. The last two decades have seen a radical change in our understanding of the interactions between the gut flora and their eukaryotic hosts and there is a growing appreciation of the spectrum of functions performed by these symbionts. Intestinal bacteria are recognized for their role in nutrient absorption, mucosal barrier function, angiogenesis, morphogenesis and postnatal maturation of intestinal cell lineages, intestinal motility and more importantly maturation of gut associated lymphoid tissue (GALT). Although gut flora are implicated in certain pathological disorders, their remarkable contributions to health and homeostasis of the host need to be recognized and understood.