Inflammatory bowel diseases occur due to an aberrant immune response to luminal antigens in genetically predisposed individuals. Although specific genetic loci have been identified underlying the predisposition, they have not fully explained the disease etiology. Striking epidemiological observations implicate the critical role of environmental influences on disease penetrance. The emergence of disease consistently observed as a society becomes modernized or developed may be attributed to westernization of diet, changing antibiotic use, or improved hygiene status. These factors are linked with changes in the gastrointestinal microbiota which, in turn, may affect development of the immune system and influence the risk of disease occurrence. Geographic variations within developing countries suggest that the strength of influence by risk factors in a society varies greatly. Studies of IBD in populations of developing countries where there are opportunities to prospectively collect changing exposure data over time may provide clues to the disease etiology.