The family contains more than 400 species of hymenopterous parasitoids in around 40 genera. It includes many important aphid parasitoids. They regulate pest aphid populations in field crops like cereals or are released on greenhouse crops. Various species are commercially produced while others have been deliberately introduced into countries against invasive aphid species.
The adults are ½-3 mm long. The female parasitoid lays an egg into an aphid nymph of a suitable development stage. The larva feeds and grows inside the aphid which continues its development and might even reach the adult stage and reproduce to a limited extend before its death. When the parasitoid larva is about to pupate, it opens a slit on the ventral side of the aphid host and constructs a cocoon attached to the leaf the host was feeding on. The pupal cocoon may be inside the aphid host (e.g. in Aphidius species) or outside (e.g. in Praon species).
The family is closely related to the family of Braconidae and is often considered to be a subfamily of it.
The following genera and individual species are currently entered under this family: