Insecticide resistance serves as an excellent example of the remarkable adaptability of insects to their environment. The development of resistance in insects is a population phenomenon and is due to selection of pre-existing genes that confer resistance. In normal populations, the frequency of individuals with genes for resistance is low. The speed of development of resistance in an insect population is dependent upon the interaction of (1) genetic (frequency, number and dominance of genes for resistance and their fitness), (2) biological (generation turnover number of progeny, refugia, migration, etc) and (3) operational (chemical nature of insecticide, insecticide persistence, life stage selected, etc.) factors. Only the operational factors can be controlled by man and hence managed to delay or prevent insecticide resistance development. The actual mechanisms of resistance vary from altered or less sensitive target sites to decreased penetration and increased metabolism. At the present time, at least 25 species of insects and spider mites that attack cotton are known to have developed resistance.