Factors inducing outbreaks of spider mites (Acari: Tetranychidae) following use of pyrethroid insecticides are reviewed. Differentials in direct toxicity between spider mites and phytoseiid (Acari: Phytoseiidae) predators provide one explanation. Wide variation exists between pyrethroids in their direct toxicity toTetranychus urticae. The acaricidal action of pyrethroids is largely controlled by the amount of irritancy or repellency induced by the respective chemicals. Laboratory assays for repellency reflect field results. Repellent activity induces spider-mite dispersal to either recolonise plants free of residues or leave the treated habitat. Pyrethroids may also affect reproduction and development rates. Residual activity is likely to vary, so limiting any general theory of spider-mite responses to pyrethroids. Variable responses to pyrethroids, strain variation and development of behavioural resistance also add complications to understanding outbreaks. Future research needs are identified.