The epidemiology of IBD is evolving. The highest incidence rates of IBD are in Europe and North America, although the overall prevalence of both CD and UC is increasing throughout the world. In particular, disease is emerging in previously low prevalence areas, such as the developing world, and among emigrant populations moving to industrialized, westernized societies. These more recent changes in reported incidence and prevalence are likely due to (1) advances in disease detection and recognition, and (2) continued environmental alterations and exposures impacting IBD onset. The hygiene hypothesis is central to our understanding of environmental exposures and their impact on the immunologic response of the gut mucosa among those genetically susceptible to IBD. These interactions between environmental and genetic influences, and ultimately their effects on the microbiota, are influencing disease onset and course. Interestingly, both protective environmental factors and risk factors have been identified in many studies. Some factors may increase the risk for one disease subtype, while reducing the risk for the other.