Mosquitoes' resistance to commonly used insecticides is now widespread, hampering control efforts and leading to substantial increases in human illness and mortality rates in many areas of the world. Insecticide bioassays are quantitative methodologies used to determine the dose-response relationship of insects to insecticides and to evaluate the susceptibility or resistance of mosquitoes to specific insecticides. They are frequently used to monitor the development of insecticide resistance in mosquitoes for both field resistance diagnoses (surveillance assays), in which the ability of mosquitoes to survive exposure to a standard dose or concentration of an insecticide is measured, and laboratory bioassays, in which responses to insecticides are tested in parallel populations of resistant (field) populations and laboratory susceptible strains using serial doses or concentrations. Metabolic detoxification, in which insecticides are metabolized by enzymes, including cytochrome P450s, hydrolases, and glutathione- S -transferases (GSTs), to become more polar and less toxic, is one resistance mechanism. Piperonyl butoxide (PBO), S , S , S -tributyl phosphorotrithioate (DEF), and diethyl maleate (DEM) are the inhibitors of P450s, hydrolases, and GSTs, respectively, and act as synergists for rapidly testing the involvement of these enzymes in insecticide resistance. Such synergistic assays are used to identify the detoxification enzyme that leads to resistance to a specific insecticide. This introduction and its associated protocols present a detailed discussion of appropriate methodologies and procedures for laboratory larval, adult, and synergistic bioassays and introduces the field surveillance tests used to monitor insecticide resistance as recommended by the latest World Health Organization (WHO) and U.S. Centers for Disease Control (CDC) guidelines.