Descriptions of the World Health Organization standard methods of assessing susceptibility or resistance in larval and in adult mosquitoes are presented, and the evaluation of their results are discussed. Other susceptibility test methods are also mentioned, including those based on esterase zymograms. Recent work on the biochemical mechanisms of resistance and cross-resistance are reviewed, along with possible countermeasures for the problem of mosquito resistance, now known in 113 species of culicines and anophelines. [References: 83]