Pesticides applied in the environment are transformed by biological or nonbiological processes into one or more transformation products. For most pesticides, transformation results in detoxification to innocuous products. Major degradation products of some currently used pesticides, however, play an important role in pest control and environmental contamination. Some pesticide degradation products are of significance in crop protection by being effective against the target pests. Some can be responsible for inadequate pest control by inducing rapid degradation of their parent compounds. Degradation products as potential contaminants of environmental and food resources has been reported recently. Although most of the currently used pesticides are biodegradable, their major degradation products should also be considered in evaluating the overall bioactivity and environmental contamination potential of the parent compound.