We reviewed the literature on aphid parasitoids to determine the occurrence, nature and outcome of intraguild interactions. Intraguild interactions were described for larval, pupal and adult aphid parasitoids and by the type of natural enemy (fungus, predator, or parasitoid). They appear to be prevalent in most aphid parasitoid systems and, except for parasitoid-parasitoid interactions, they are mostly asymmetric, with aphidophagous predators and pathogens killing parasitoids. The limited experimental evidence from field studies is insufficient to provide a comprehensive pattern of the consequences of intraguild interactions for aphid parasitoid populations in general and, more specifically, for the efficacy of biological control. However, because intraguild interactions are widespread in aphid-natural enemy communities and mostly detrimental to aphid parasitoids, we conclude that intraguild interactions have a primary effect in driving fluctuations in aphid parasitoid populations. Drawing on case studies, we further argue that intraguild interactions can substantially alter the effectiveness of aphid parasitoids as biological-control agents.