Spray drift from application sites, runoff from agricultural fields, leftover products from home use, and accidental spills have made pesticide contamination ubiquitous in the environment. As a pesticide moves through the environment, it may react through chemical and biotic processes such as hydrolysis, oxidation, or reduction, or be metabolized in microorganisms, animals, plants, and humans. Most reactions will be inactivations, forming degradation products less toxic or persistent than the parent compound. However, some reactions are activations, creating breakdown products equally or more toxic, persistent, or mobile than the parent and posing a greater threat to nontarget organisms and the environment. Examples are drawn from the major classes of pesticides including organochlorine compounds (DDT and aldrin), organophosphorus pesticides (malathion), carbamate pesticides (aldicarb), and fungicides to illustrate the various activation routes.