In so far as numbers of species are concerned, this family is one of the largest of the Chalcidoidea (along with Pteromalidae and Encyrtidae). It is among the most difficult of all groups to learn because of the large numbers of species and because most often specimens are lightly sclerotized and, unless critical point dried, they shrivel badly. Because of this it is difficult to see some characters on eulophids even though the characters are good. For instance, it is not always easy to count tarsal or antennal segments in specimens that have been air dried. The difficulty of working with these specimens has also kept many workers from venturing into the group.

The family is divided into 4 to 5 subfamilies, some of which have yet to be rigorously defined. In fact, the primary character used most often in the literature to separate them (a break in the submarginal vein) occasionally does not work and is difficult to see even when present. We have avoided this character in the pictorial key and have tried to emphasize those that are easier to see. As a result, our key proceeds a little differently than others. The best keys to subfamilies and genera are Schauff, LaSalle, and Coote (in Gibson, et al. 1997), LaSalle (1994), Schauff (1991), Peck, Boucek, and Hoffer (1964), Graham (1987, 1991) and Boucek (1988). Neotropical eulophids are not well studied, but Boucek (1977a) and LaSalle and Schauff (1992, 1995) have made a start.

Eulophinae: About 150 Nearctic species in 33 genera. Miller (1970) gave a key to genera of Eulophini (which he treated as a subfamily) for North America. He reviewed the genera Pnigalio and Sympiesis. Dicladocerus was revised by Yoshimoto (1976) and Diglyphus was reviewed by Gordh and Hendrickson (1979). The genus Zagrammosoma was reviewed by Gordh (1978), and Schauff revised Hyssopus (1985b) and Elachertus (1985c).

Euderinae: 35 Nearctic species in 6 genera. Coote (1994) has revised the genera for North America. The primary genus, Euderus, was revised by Yoshimoto (1971).

Tetrastichinae: 212 Nearctic species in 42 genera (LaSalle 1994). Graham's monumental works on this subfamily (1987, 1991) have drastically altered the generic names applied to the majority of the species. LaSalle's (1994) key to the genera is the most detailed yet published for North America and also provides a complete listing of the included species. Boucek (1977b) provided a tentative key to genera (Tetrastichinae, sensu Boucek), but did not include all the North American groups. Tetrastichus (now mostly Aprostocetus) was revised by Burks (1943), but undescribed species are legion and could possibly double or triple the known number in the Nearctic Region. The species are not easily identified. Dahms (1984a) revised Melittobia. He also has reviewed their biology and added observations of his own (Dahms 1984b).

Entedoninae: About 145 Nearctic species in 27 genera; the Hymenoptera catalog (Burks, 1979) is hopelessly outdated in this group, due to a number of recent reviews. About half the currently recognized genera contain a single species each, so that the possibilities of new generic recombinations are ever present. Papers by Yoshimoto cover the genera Chrysocharis (subgenus Kratochviliana 1973a and Chrysocharis 1973b) and Chrysonotomyia (1978b, 1980) which replaces the better known Achrysocharis, Achrysocharella, and Derostenus (of American authors). Schauff (1985a, 1988) has revised New World Paracrias and North American Entedon. Hansson revised Nearctic Closterocerus (1994a) Chrysonotoymia, Telopterus (1994b), Neochrysocharis (1995a) and Omphale (1996). He has also reworked Chrysocharis (1995b), Chrysonotomyia and Neochrysocharis for Europe (1985, 1990) and the New World (1987), Derostenus for most of the world (1986b) and Zaommomyia (1986a). Hansson (1988b) has recorded species of Holcopelte and Ionympha from North America. LaSalle and Schauff (1994) described the new tribe Euderomphalini.

Melittobia (Tetrastichinae): A parasite of aculeate Hymenoptera (bees wasps), Melittobia is strongly sexually dimorphic. In addition there are 2 generations of dimorphic forms which develop on a single host larva. The first brood has 2 forms, 1 feeds on haemolymph, has females which are weakly sclerotized, brown and with nonfunctional wings, and males with vestigial or absent ocelli, unpigmented eyes, and short wings. The second form feeds on solid material and has females which are heavily sclerotized, black, and with functional wings and males with ocelli, pigmented eyes, and normal wings. The haemolymph form lays a second batch of eggs on the same host immediately upon reaching adulthood (14 days), whereas the solid feeding forms continue to feed for 90 days. The progeny of the haemolymph form feed on solid tissue and develop as a second generation of "normal" morphs alongside the first.

Euplectrus (Euplectrini: Eulolphinae): The species in this genus are gregarious external parasites of lepidopterous larvae. It is particularly interesting in 2 ways: first, the larvae feed on the dorsum of their host which is often still moving freely about (thus, if you see a free-living caterpillar with larvae on its back, the larvae are almost certainly Euplectrus); secondly, the larvae move below the emaciated host to pupate, spinning silk from the Malphigian tubes to hold the host down as well as to separate themselves from each other. According to Askew (1971) some other chalcidoids spin cocoons (e.g. some Coccophagus-Aphelinidae, Systasis-Pteromalidae, some Encyrtidae), but it is uncommon.

Aprostocetus (Tetrastichinae): Two species in this genus (from Europe) are predators of gall-mites. These are the only known chalcidoids which attack mites (exclusive of ticks). Some species of Tetrastichus are phytophagous.

Psephenivorus (Entedoninae): This genus contains the only species of aquatic eulophid. It attacks psephenid prepupae under water (Burks 1968b).

Thrips parasites: Apparently Eulophidae is one of only 2 hymenopterous families (trichogrammatids being the other) which parasitize Thysanoptera. Members of the genera Ceranisus, Thripoctenoides, Goetheana (all Entedoninae), and Tetrastichus (Tetrastichinae) are known to attack thrips larvae (trichogrammatids attack the eggs).

Whitefly parasites: The tribe Euderomphal-ini is unique within the family in that all members so far described are parasites of whiteflies (LaSalle and Schauff 1994).