Crohn's disease and ulcerative colitis are immune-mediated diseases of the gastrointestinal (GI) tract, which are conditions caused by an overactive immune system. There are rising cases worldwide. In this Massachusetts General Hospital Blum Center presentation from May 19, 2023, to celebrate World Inflammatory Bowel Disease Day, Kristin E. Burke, MD, MPH, reviews what is known about how these diseases develop. She also provides updates on the expanded treatment options currently available for these diseases.