The way scientists think about how chemicals cause their toxic effects is changing. Recent scientific research tells us that the traditional notion of how chemicals act is being replaced by a better understanding of the actual features of exposures to environmental chemicals. These features include the timing and vulnerability of exposures, exposures to mixtures, effects at low doses and genetic alterations called epigenetics. Traditional thinking tells us that how much of a chemical you are exposed to (the dose) determines the effect. This principle assumes that chemicals act by overwhelming