Agricultural applications of the foliar-applied pyrethroid insecticides can result in small quantities of these products entering aquatic environments by spray drift deposition or by runoff. A number of mesocosm studies have been carried out with these products to help quantify the effects of this entry on aquatic organisms and ecosystems. However, there has been relatively little detailed scientific consideration of the amounts of these chemicals entering aquatic environments during normal agricultural usage, nor of how to translate field aquatic exposure data into mesocosm treatment rates and procedures. Therefore, doubt is inevitably cast upon environmental risk assessments made with the results from these aquatic biological studies. This chapter assesses spray drift deposition and runoff for foliar-applied pyrethroids, using data from published and unpublished sources. Although at present there is a paucity of relevant information, the available data are discussed in order to derive a best estimate of application numbers, frequency, and rates of pyrethroids for mesocosm investigations.