Ticks rank first as disease transmitting agents to non-human vertebrates and next to mosquitoes as vectors of infectious disease agents to humans. Rhipicephalus (Boophilus) microplus,is a voracious blood sucker and transmits babesiosis and anaplasmosis.Traditional control methods for these ectoparasites are mainly based on use of chemical acaricides like synthetic pyrethroids, macrocyclic lactones etc.Development and market production of a chemical compound takes a long time and is very expensive; this is aggravated by non-availability of other effective control methods. The situation being made worse by indiscriminate and frequent use of acaricides which exerts selection pressure on tick population. During the past twenty years there are reports of emergence of resistance in R. microplus in several countries.Resistance against synthetic pyrethroids arises either due to target site insensitivity or increased metabolism of the acaricide by the insects.This book highlights the importance in assessing the level of pyrethroid resistance in these ticks and the mechanisms involved there in with main emphasis on susceptibility to synthetic pyrethroids and role of esterases.