Consistency of control is a key limitation in the use of biological pest control agents. This is especially important in case of biocontrol agents that are used against soil inhabiting pests due to the great complexity of the soil environment. Information on the ecology of the biocontrol agent can provide powerful tools for enhancing predictability of pest control with biocontrol agents. Entomopathogenic nematodes in the families Heterorhbditidae and Steinernematidae are important biocontrol agents of soil insect pests. These nematodes are used mostly in innundative biological control, but inconsistency in pest control from test to test can reduce grower confidence and frustrate implementation efforts. Carefully designed studies on the ecology of entomopathogenic nematodes now provide tools to enhance the predictability of pest control. Studies on host finding behavior, host attack and penetration behavior, coupled with host defense strategies has led to better matching of nematode species with target hosts. Such studies have also led to ways that can be used to suppress host defenses or increase their susceptibility to nematode parasites. Furthermore, an enhanced understanding of the biotic and abiotic factors, including genetic variation in virulence and stress tolerance, affecting natural populations of entomopathogenic nematodes now provide useful tools to enhance success of biological control.