The systematics, biology and distribution of the Aphidiinae in Sweden were studied since 1978. Colonies of various aphid species were reared in vials. Approximately 2000 rearings were undertaken and all emerging insects were counted and identified. To elucidate the ecological relationships between aphids, parasitoids, hyperparasitoids and predators, all data from the rearings will be computerized. A preliminary sketch of the relationships (not based on a computer analysis) is presented. Hitherto, apprx 100 spp. of Aphidiinae have been found in Sweden. The usefulness of utilizing host data in Aphidiinae systematics is discussed.