Pyrethroid insecticides have been used extensively in controlling arthropod pests and disease vectors. They act on voltage-gated sodium channels, which are essential for the initiation and propagation of the action potential in neurons and other excitable cells. A serious obstacle to effective use of these compounds is the emergence of resistance. One major mechanism of DDT and pyrethroid resistance is known as knockdown resistance (kdr). Numerous mutations in sodium channels have been identified that confer resistance to pyrethroids in various arthropod pests and disease vectors. Identification of kdr mutations not only provides precise molecular markers for rapidly assessing the frequency of resistance alleles in field populations, but has also proven to be extremely valuable for elucidating the molecular identity of pyrethroid receptor sites, PyR1 and PyR2.