Inflammatory bowel disease (IBD), including Crohn’s disease (CD) and ulcerative colitis (UC), are chronic debilitating diseases that occur in populations around the world. As these conditions diseases can manifest at any age, and therefore represent a clinical challenge for pediatricians, internists, family practitioners, and surgeons. The underling etiology is thought to be multifactorial. There is a well-defined genetic contribution to the diseases, but this does not fully explain the epidemiology. Environmental factors, including the composition of the gut microbiota, are also important. Dr Jim Lewis sets the stage by overviewing the epidemiology of IBD. Dr Lewis also explores the relationships between IBD and other environmental factors, such as diet, infections, and medications.