This article summarizes available literature regarding the postnatal development of the gastrointestinal (GI) system. Where possible, the timing of structural, morphological, chemical and functional development is compared between common laboratory animal species. Postnatal development of the stomach, small intestine (SI), and large intestine is reviewed, and a brief discussion of the postnatal development of the pancreas and liver, which are both important accessory organs to digestion, is included. Each section begins with a general description of the structure of each organ, followed by a discussion of human postnatal development for each organ system. This review is part of the initial phase of a project undertaken by the Developmental and Reproductive Toxicology Technical Committee of the Institute for Life Sciences International (ILSI) Health and Environmental Sciences Institute (HESI) to bring together information on a selected number of organ systems and compare their postnatal development across several species (Hurtt and Sandler, 2003).