An overview of the major phase 1 and phase 2 reactions involved in the metabolism of insecticides in mammals is provided. The metabolism of some important selected insecticides within each of the major historic and current use classes of insecticides is described: organochlorine insecticides (including the chlorinated diphenylethanes and the chlorinated cyclodienes), organophosphorus insecticides, N-methylcarbamate insecticides, pyrethroid insecticides and neonicotinoid insecticides. The impacts of metabolism on the acute toxicity levels of the insecticides as well as the likelihood of the insecticides or their metabolites to bioaccumulate are also discussed.