The volume deals with morphology, taxonomy and systematics of entomopathogenic nematodes (EPN) in the families Steinernematidae and Heterorhabditidae and bacteria associated with these nematodes. In the first part, history, taxonomic status, family and genus definitions of EPN are mentioned. In the second part, all useful aspects of morphology and methodology of EPN are mentioned with illustrations and SEM photographs and instructions on how to make the measurements, and how to use them in taxonomic work. A polychotomous key with SEM photographs and illustrations is provided. In the third part, full descriptions of all species are presented with latest information about each species. In the fourth part, molecular and phylogenetic methods for working with EPN are presented, including the latest information and instructions on how to use molecular data in taxonomic work. In the last part, bacteria associated with this group of nematodes are discussed in the context of the latest information about methodology, biology and taxon.