Complications in inflammatory bowel disease determine the severity of disease as well as the complexities of medical or surgical treatment opportunities. Therefore, in known inflammatory bowel disease, the prevention, the early detection and the adequate therapeutic response to certain complications are important goals in the follow-up of inflammatory bowel disease patients. Disease complications are separated into intestinal and extraintestinal complications. Intestinal complications are somewhat disease specific, which means that they occur exclusively in either Crohn's disease or ulcerative colitis (e.g., enteric fistulas are particularly found in Crohn's disease and toxic megacolon in ulcerative colitis). Most extraintestinal complications occur in both forms of inflammatory bowel disease (e.g., anemia, thromboembolic events or osteoporosis). The current knowledge on pathogenesis, diagnostic tools, prevention and treatment of certain intestinal and extraintestinal complications is reviewed.