Many animal species are still undiscovered, even in Sweden. This thesis deals with the wasps of the subfamily Eucoilinae (of Figitidae, Cynipoidea), a poorly known group of small parasitoids of Dipteran flies. In this group, old classifications are chaotic, knowledge of old taxa is scant, phylogenetic work has recently started, and many new taxa remain to be described. In this thesis, an identification key is given to the European genera of Eucolinae, and a tribal classification of the subfamily is reintroduced, based on recent phylogenetic results. Most of the thesis focuses on the tribe Eucoilini. That tribe is analyzed phylogenetically in two analyses, one based on a large morphological dataset, the other on two molecular markers, ribosomal 28S and mitochondrial Cytochrome B. The monophyletic core group of Eucoilini is the Eucoila/Trybliographa complex. It is treated in a big taxonomic revision, based on studies of several hundred possibly relevant type specimens and several thousands of other specimens, and on the phylogenetic analyses. More than 500 names of possible members of this group are investigated, and many new combinations are proposed, of species found not to belong within the group, as well as those that did, including some which are transferred to other genera in accordance with the new phylogenetic results. The new genus Quasimodoana is erected, and 20 new species described (8 from Northern Europe, 12 from elsewhere). More than 180 new combinations, more than 50 new species-level synonyms and ca 10 genus-level synonyms, are proposed. Within this complex, the basal split is found to have been between a Paleotropic, specis-poor lineage (Bothrochacis), and a species-rich and largely Holarctic lineage, dominated by Trybliographa but also including the small genera Eucoila and Linoeucoila. The three are not satisfactorily resolved in the analysis, but still maintained as separate genera for pragmatic reasons. It is hypothesized that this group originated in Asia during the Eocene or Oligocene, attacking calyptrate flies in herbivore dung. Then it separated in a tropical branch retaining the ancestral life history mode, and a Holarctic branch, where some lineages specialised in Anthomyiidae and colonised the microhabitats of fungi and plants, speciating extensively in the boreal forests.