The exact cause of inflammatory bowel disease remains undiscovered, but its destructive nature is clearly recognized. In this article, the authors summarize what is known about the pathogenesis and epidemiology of ulcerative colitis and Crohn's disease and how to distinguish between the two, both clinically and pathologically. They also describe disorders that may mimic inflammatory bowel disease. In part 2 of this article, beginning on page 86, the authors discuss some of the therapeutic options that are the commonly used as well as some that are investigational but show promise for patients with this chronic and relapsing disease.