The developing gastrointestinal tract from conception to adolescence is in constant direct interaction with an increasingly complex environment. This sets up the potential for unrecognized acute as well as chronic disorders, some of which may be difficult to pinpoint in a developing infant and child, given the wide variations that exist. It is startling to note how early some environmental toxins can come into contact with the developing human, where vulnerability may be heightened and maturation of detoxifying pathways may be incomplete. Although the complex process of recognizing, detoxifying, and avoiding the toxic substance by the body has presumably evolved over a substantial period of time, in this rapidly changing world, the array of novel toxins that make their way into the gastrointestinal tract is increasing. There remain many gaps in understanding the effects of environmental toxins on all of the developmental stages from conception to adolescence. Although threshold levels have typically been derived from adult or animal data, factors such as size, relative differences in consumption in proportion to size especially in infancy, and variable physiologic maturation of metabolic pathways are not well understood. The vulnerability may be further accentuated by physical factors that alter with maturity, such as permeability and critical times during organogenesis or organ maturation. Also of concern is how little is known about low-dose, long-term exposure, as well as any interplay with common illnesses. This article focuses on environmental toxins that have been shown to have toxic effects on the gastrointestinal tract.