Inflammatory bowel diseases (IBD) encompass Crohn's disease and ulcerative colitis, which are diseases characterized by chronic intestinal inflammation. IBD is believed to result from predisposing genetic and environmental factors (specific antigens and pathogen-associated molecular patterns) acting on the immunoregulatory system and causing inflammation of the gastrointestinal mucosa. IBD may be the result of an imbalance of effector (proinflammatory) and regulatory T-cell responses. Three scenarios indicative of the outcome of this balance exist in animal models: balanced effector and regulatory T cells resulting in a normal controlled inflammation; overactive effector T cells resulting in inflammation and disease; and an absence of regulatory T cells resulting in uncontrolled inflammation and severe, aggressive disease. The number of products under study for the treatment of IBD has increased from 3 products and 1 target in 1993 to more than 30 products and more than 10 targets in 2005. The number of products under development and continued investigations into the pathogenesis of IBD emphasize the need to expand clinical research efforts in IBD.