Abstract: This paper is the fourth part of a review on recent progress in insectmolecular biology. It summarizes the current understanding of the molecular basis underlying insecticide resistance conferred by enhanced detoxication (through activities of hydrolases, cytochrome P450, glutathione S-transferase) and by reduced target sensitivity (changes in AchE, sodium channel, GABA receptor). At least some of the molecular mechanisms of resistance, namely gene amplification, point mutation, and trans-acting regulatory element, have been proven correct.