The use of entomopathogenic nematodes (EPNs) is a safe and eco-friendly alternative tosynthetic pesticides for the control ofsoil and above-ground insect pests in the world. The major objective of this review was to summarize the available publications concerning the virulence of different EPNs and their bacterial symbionts on several insect pests leading to death. In this context, different criteria had been discussed, such as the correlation between insect mortality and EPN concentration and time of exposure, the sensitive stage of insects to EPN, variation of the pathogenicity of various EPNs, and significance of the insect mortality. The impact of EPNs on some biological and physiological processes of the insect host hasbeen reviewed. The present review, also, highlighted the key factors and environmental conditions interfering with the EPNs' virulence against insect pests, in addition to the application techniques of EPNs and important precautions. In view of this discussion, many insect pests, in the soil or above ground, may be successfully controlled by using effective EPNs under suitable biotic and abiotic environmental conditions, as well as with appropriate EPN concentration and formulation.