Pyrethroid insecticides are a potent group of chemicals used to control insect pests in agricultural and aquatic systems. The mode of entry of pyrethroids into aquatic habitats is both direct and indirect. Direct sources include purposeful applications of pyrethroids in vector control as well as agricultural and silvicultural pest control programs. Indirect means through which water bodies could be contaminated with pyrethroid residues include spray drift, runoff and erosion. In field studies, the non-target aquatic groups most affected were the Ephemeroptera, Plecoptera, Odonata, Hemiptera, Coleoptera and Trichoptera. The population of affected organisms, however, generally recovered to pretreatment levels within weeks to months after application. The transient effects of pyrethroids on some non-target species will have a short-term impact on the densities and feeding behavior of some dependent carnivorous fish species in aquatic ecosystems. Fish species like brook trout and slimy sculpins utilize alternate food sources when aquatic insects became unavailable. The susceptibility of aquatic insects may be related to the biochemical and physiological constraints associated with aquatic life. Pesticide bioassays require standardization to allow for more meaningful comparisons between studies. Further understanding of the processes that influence the susceptibility of aquatic organisms to pyrethroid insecticides is critical for the effective and safe use of these compounds in areas adjacent to aquatic environments.