In several laboratory and field strains of insects, the resistance to DDT or to dieldrin has been deduced to be due to a single gene allele. In certain other strains, resistance has been nonspecific and considered to be due to vigor tolerance. The dosage-mortality curves of successive generations have been shown to be of use in determining whether resistance is unifactorial or polygenic in origin. Examples of these two types are drawn from Drosophila, Musca, and several species of Anopheles. The relation of DDT-dehydrochlorinase to the genetic origin of DDT resistance and the significance of inducing DDT resistance and tolerance by means not involving DDT are discussed. The existence of multiple modifier genes, which may be partially congruent with vigor-tolerance genes, is indicated by the rapid redevelopment of resistance by reverted strains. Gene alleles for resistance may be pleiotropic and may confer extra susceptibility to certain “negatively correlated” compounds. The relation of visible chromosome inversions to resistance genes is discussed. It is submitted that full understanding of the genetic nature of the various resistances depends on a knowledge of the biochemical mechanisms involved. The above considerations will aid in foretelling the consequences of insecticide selections pressure on an arthropod species.