Pyrethroid insecticides, from natural pyrethrins to photostable analogues, represent important weapons against insect pests of both economic and medical importance. They share many characteristics with DDT, including a negative temperature coefficient, and knockdown and killing activity resulting from action against the sodium channels of the peripheral and central nervous systems. In this review, Eduardo Zerba summarizes what is known about their penetration into target insects, their mode of action, metabolism and excretion.