Safety and environmentalcal insecticide issues surrounding the use of chemical insecticides has led to an emphasis on developing alternative control measures such as entomopathogens and their products. Entomopathogenic nematodeare effective biopesticide which can be incorporated in IPM programs because they are considered non-toxic to humans, relatively specific to their target pests and can be applied with standard pesticide equipment. Entomopthogenic nematodes have proven to be the most effective as biological control organisms. Entomopathogenic nematodes have been released extensively in crop fields with negligible effects on non target insects and are regarded as exceptionally safe to the environment. Our focus in this paper was to review mechanism and pathogencity of nematode, phylogeny of nematode for Steinernematidae and Heterorhabditidae. Steinernematidae is represented by the genera Steinernema and Neosteinernema and Heterorhabditidae is represented by the genus Heterorhabditis. They are associated with mutualistic bacteria in the genus Xenorhabdus for Steinernema and Photorhabdus for Heterorhabditis. Thus, it is a nematode bacterium complex that works together as a biological control unit to kill an insect host by penetrating the host through natural opening and there by releasing the bacterial symbiont which spread and multiply in the haemolymph of the insect pest and kill them by septicemia. Infective juvenile entomopathogenic nematode locate their hosts in soil by means of two strategies-ambusing and crusing. Nematode employs different foraging strategies to locate and infect hosts. Genetic diversity may be lost, or genetic variation may have been limited during collection or lost during importation and rearing. A serious problem for EPNs is founder effect because only a limited number of insect cadavers are collected at single geographical sites, resulting in reduced genetic variance. EPNs have been most efficacious in habitats that provide protection from environmental extremes, especially in soil, which is their natural habitat and in cryptic habitats. Excellent control has been archived against plant-boring insects because their cryptic habitats are favorable for nematode survival and infectivity. In developing biocontrol programs using EPNs, one mechanism to increase the chance of success is to screen novel nematode species or strains for potential efficacy against particular target pests.