Inflammatory bowel disease (IBD) encompasses ulcerative colitis (UC) and Crohn’s disease (CD). Both conditions cause chronic relapsing inflammation in the gastrointestinal (GI) tract, but have different characteristics. UC causes diffuse mucosal inflammation limited to the colon, extending proximally from the anal verge, with the rectum involved in 95% of patients. UC is described in terms of the disease extent: proctitis (confined to the rectum), proctosigmoiditis (disease confined to the recto-sigmoid colon), distal disease (distal to the splenic flexure), and pan-colitis (the entire large intestine). The extent of disease can change, with proximal extension seen in approximately a third of patients with proctitis, although there is great variation between studies. CD causes inflammation that can affect the entire thickness of the wall of the intestine, and is not confined to the mucosa. CD can affect any part of the GI tract. The terminal ileum is affected in approximately 80% of cases, the colon in approximately 60% of cases, and the rectum and perianal region in approximately 40% of cases. CD is classified by location (ileal, colonic, ileocolonic, upper GI tract), by the presence of stricturing or penetrating disease, and by the age of onset (before or after the age of 40). Penetrating disease refers to the development of fistulae, which can lead to complications such as abscesses or perforations. An earlier age at onset is associated with more complicated disease. The diagnosis of UC or CD is established through a combination of clinical, endoscopic, radiological, and histological criteria rather than by any single modality. Occasionally, it is not possible to establish an unequivocal diagnosis of CD or UC in IBD, and a third category, accounting for nearly 10% of cases, is used, termed IBD unclassified.