Pyrethrins and synthetic pyrethroids are used extensively as insecticides and acaricides for the treatment of a broad range of ectoparasites in large and small animals, as well as in nonmammalian species such as birds, fish, and honeybees. These compounds are used in veterinary medicine in different formulations including spot-on, sprays, ear tags, dips (immersion), soluble powders, and shampoos to control fleas, mite, lice, and ticks between other insect infestations both outside and inside the house. The synthetic pyrethroids have been classified in two classes: type I and type II; the addition of the alpha-cyano group to the 3-phenoxybenzyl alcohol group in type II has increased the insecticidal potency. The mode of action of these compounds suggests that the voltage-dependent sodium and chloride channels, mitochondrial damage, and oxidative stress in the nerve membrane are the common targets in insects and mammals. Moreover, they act via modulation of nicotinic cholinergic transmission, as well as decrease dopamine and serotonin contents in the Central nervous system (CNS). The pharmacokinetic/toxicokinetic properties of these compounds are also presented. This chapter also reviews the antiparasitic activities and the veterinary applications (uses) of pyrethrins and pyrethroids in several animal species. The clinical signs of poisoning, in particular, in the cats by permethrin are described. The chapter also provides the European Union’s (EU) maximum residue limits (MRLs) established for the pyrethroids as antiparasitic agents in food-producing animals; the MRLs are necessary to establish the withdrawal/withholding periods of the final veterinary drug formulations containing these chemical compounds.