AbstractThe first two sections of this chapter deal with predators and parasitoids, respectively, with focus on the foraging behaviour and on cues triggering successful search. Laboratory studies suggest that many natural enemies of aphids have the potential to decimate the host or prey population. Because of low search success, however, they often fail to do so in the field, or fail to do so in a predictable manner. The third section on microbial agents focuses on parameters affecting the epizootiology, i.e. the potential of a pathogen to cause a population regulating epidemic or epizootic. The relationship between host or prey specificity and resource utilization in terms of enemy effectiveness is also discussed. In the fourth section, mutualistic relationships and intraguild predations are considered. Several recent studies have shown that ant-aphid mutualism and intraguild predation may limit the effectiveness of Aphidophaga as biocontrol agents in the field.