Pyrethroid insecticides were first introduced into India in 1980, primarily for the control of Spodoptera litura, which by that time had become resistant to organophosphate, carbamate and organochlorine insecticides (Ramakrishnan et. al., 1984). Within few years, pyrethroids became popular with cotton farmers and were indiscriminately used and replaced all other insecticides (Reddy, 1987). Resistance of the cotton bollworm, Helicoverpa armigera to pyrethroids was first reported in Australia (Gunning et al, 1984) and later it was reported in many other countries. In India, pyrethroid resistance was first noticed between “1984-1985” in the southern region of India (Dhingra et. al., 1988 & McCaffery et. al., 1988) and later it was reported in many other regions in the country. Three mechanisms of resistance to pyrethroids have been identified in H. armigera, including enhanced nerve insensitivity and reduced penetration. The mechanism of reduced penetration holds more toxicant outside the insect body whereas metabolic mechanisms degrade the penetrated compound. Thus a lethal quantity of the toxicant may never accumulate at the target site where the toxic threshold level of the insecticide may itself be higher if a mechanism of target-site sensitivity is also present. The flexibility of H. armigera to use a variety of modification in all of their resistance mechanisms, make it difficult or less easy to precisely identify the nature of resistance.