Chronic inflammatory bowel diseases, ulcerative colitis and Crohn's disease, are thought to be distinct clinical entities. The mechanisms involved in the pathogenesis of the diseases are poorly understood, however, it is thought that dysregulation of the immune system, especially of the mucosal immune reactivity, as well as a genetic susceptibility and environmental factors, e.g. bacterial flora, may each contribute to initiation and/or progression and chronicity of the diseases. Experimental animal models are available to study the different components involved in inflammatory bowel disease and to complement our understanding of chronic inflammatory reactions in these organs ultimately leading to effective alternative therapeutic approaches.