Specific targeting of drug to the colon is recognized to have several therapeutic advantages. Drugs that are destroyed by the acidic environment of the stomach or metabolized by pancreatic enzymes are only slightly affected in the colon. Sustained colonic delivery of drugs can be useful in the treatment of nocturnal asthma, angina, and arthritis. Treatment of colonic diseases such as ulcerative colitis, Crohn's disease, and colorectal cancer is more effective with direct delivery of drugs to the colon. This article is aimed at providing insight into the design considerations and evaluation of colonic drug delivery systems. The anatomy and physiology of the lower gastrointestinal tract is discussed; then the biopharmaceutical aspects are considered in relation to drug absorption in the colon. Various approaches to colon-specific drug delivery are discussed.