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<italic>Cosmospora</italic> (in the broad sense; Nectriaceae, Hypocreales, Ascomycota) are fungi that parasitize other fungi, particularly fungi in the Xylariales (Ascomycota), or scale insects. Morphologically, these fungi are known for having one of the most simplest and smallest sexual fruiting bodies (<300 μm) among the Nectriaceae. The sexual spores are generally warted. The majority of <italic>Cosmospora</italic> species have acremonium-like or fusarium-like asexual states. The name <italic>Cosmospora</italic> is derived from the ornamentation in the sexual spores (Gr. <italic>cosmos</italic> = ornamented + Gr. <italic>spora</italic> = spore). The main goals of this dissertation were to revise Cosmospora sensu stricto, and to determine the evolutionary relationship between <italic>Cosmospora</italic> species and their associated fungal hosts. Additionally, <italic>Corallomycetella</italic> (Nectriaceae, Hypocreales, Ascomycota), a lineage basal to <italic>Cosmospora</italic> sensu lato, was revised as well. Molecular and classical taxonomic tools were used to revise the genera. A genus was recognized if the clade met the following criteria: 1) the clade was well supported, 2) the clade was associated with a unique asexual state, and 3) the clade was ecologically different. A species was recognized if the clade met the following criteria: 1) the clade was well supported in the majority of single gene trees, 2) the clade was morphologically different, and/or 3) the clade was ecologically different in regards to host. <italic>Cosmospora</italic> species were observed to be highly host specific. Thus, host was recognized as an important character to delineate species, and the host specificity led us to hypothesize that <italic>Cosmospora</italic> species and their associated hosts were cospeciation (i.e., their association was not random). Two new genera, nine new combinations, and eleven new species were described in the taxonomic work included in this dissertation. A significant global congruence was determined between the <italic>Cosmospora</italic> and host phylogenies. However, host-switch events seemed more abundant in the early lineages of the host, while cospeciation events seemed more common in more recent lineages of the host. This phylogenetic signature is consistent with pseudocospeciation, but it could not be confirmed given that divergence estimates could not be estimated.