Desjardins, Christopher AChalcidoidea, one of the largest superfamilies of parasitic Hymenoptera, has major importance in the biological control of insect pests. However, phylogenetic relationships both within and between chalcidoid families have been poorly understood, particularly within Pteromalidae, one of largest families. This study approaches the problem of pteromalid phylogeny from two directions, coupling a detailed morphological revision of one of the most divergent and poorly-known subfamilies of pteromalids (Diparinae) with a broad, exemplar-based molecular study that seeks to place this subfamily in the broader context of pteromalid and chalcidoid phylogeny. First, a morphological phylogenetic analysis of the world genera of Diparinae is provided based on 76 characters. Diparinae is supported as monophyletic based on the presence of a cercal brush in all analyses. The cercal brush, in combination with the absence of a smooth, convex dorsellum, is diagnostic for Diparinae. <i>Liepara</i> Boucek (Pteromalidae) and <i>Bohpa</i> Darling (Pteromalidae: Ceinae) both appear as sister-groups to Diparinae in different analyses. The phylogenetic analysis is used to develop a new classification scheme, under which Diparinae consists of 116 species in 14 genera. Three genera and 14 species are described as new, and a key to all genera is provided. Second, forty-two taxa broadly representing Chalcidoidea and more specifically Pteromalidae were sequenced for 4620 bp of four nuclear protein-coding genes, including 1719bp of CAD, 708bp of DDC, 1142bp of enolase, and 1044bp of PEPCK. The combined data set was analyzed using maximum likelihood methods, and the AU test was used to test support for non-monophyly of taxonomic groups which appeared para- or poly-phyletic in the tree. Phylogenetic relationships that have been supported by previous morphological and molecular evidence were recovered (e.g., monophyly of Chalcidoidea), as was the monophyly of groups well supported by morphology but resolved as polyphyletic in previous molecular analyses (e.g., Chalcididae). The monophyly of Pteromalidae and the pteromalid subfamily Colotrechninae are both strongly rejected (p&#60;0.001). New hypotheses are proposed for relationships within Chalcidoidea, including Eutrichosomatinae (Pteromalidae) as the basal lineage of the perilampid/eucharitid clade. This study demostrates that molecular and morphological data can provide reciprocal illumination for understanding relationships within Chalcidoidea.en-USDissertationBiology, EntomologyBiology, Molecular