Ulcerative colitis and Crohn's disease are inflammatory disorders of the gastrointestinal tract that are distributed unevenly within populations and throughout the world. Although the exact causes of inflammatory bowel disease (IBD) remain unknown, study of the epidemiology of IBD has provided insight into pathogenesis. This article examines the geographic, ethnic, and other trends of IBD; risk factors (including genetic and environmental); and the natural history of IBD.