Ulcerative colitis and Crohn's disease are inflammatory disorders of the ga
strointestinal tract that are distributed unevenly within populations and t
hroughout the world. Although the exact causes of inflammatory bowel diseas
e (IBD) remain unknown, study of the epidemiology of IBD has provided insig
ht into pathogenesis. This article examines the geographic, ethnic, and oth
er trends of IBD; risk factors (including genetic and environmental); and t
he natural history of IBD.