At present 75 species of Hypocrea have been identified in temperate Europe. Nineteen green-spored species and their Trichoderma asexual states are here described in detail. Extensive searches for Hypocrea teleomorphs in 14 European countries, with emphasis on Central Europe, yielded more than 620 specimens within five years. The morphology of fresh and dry stromata was studied. In addition, available types of species described from Europe were examined. Cultures were prepared from ascospores and used to study the morphology of cultures and anamorphs, to determine growth rates, and to extract DNA that was used for amplification and sequencing of three genetic markers. ITS was used for identification, while RNA polymerase II subunit b (rpb2) and translation elongation factor 1 alpha (tef1) were analyzed for phylogenetic reconstruction of the genus.SEVERAL UNEXPECTED FINDINGS RESULTED FROM THIS PROJECT: 1) The previous view that only a small number of Trichoderma species form a teleomorph is erroneous. 2) All expectations concerning the number of species in Europe are by far exceeded. Seventy-five species of Hypocrea, two species of Protocrea, and Arachnocrea stipata, are herein identified in temperate Europe, based on the ITS identification routine using fresh material, on species described earlier without molecular data and on species recently described but not collected during this project. 3) Current data suggest that the biodiversity of Hypocrea / Trichoderma above soil exceeds the number of species isolated from soil. 4) The number of Trichoderma species forming hyaline conidia has been considered a small fraction. In Europe, 26 species of those forming teleomorphs produce hyaline conidia, while 42 green-conidial species are known. Three of the detected Hypocrea species do not form an anamorph in culture, while the anamorph is unknown in four species, because they have never been cultured.This work is a preliminary account of Hypocrea and their Trichoderma anamorphs in Europe. Of the hyaline-spored species, H. minutispora is by far the most common species in Europe, while of the green-spored species this is H. strictipilosa.General ecology of Hypocrea is discussed. Specific associations, either with host fungi or trees have been found, but the majority of species seems to be necrotrophic on diverse fungi on wood and bark.The taxonomy of the genus will be treated in two parts. In this first part 19 species of Hypocrea with green ascospores, including six new teleomorph and five new anamorph species, are described in detail. All green-spored species belong to previously recognised clades, except H. spinulosa, which forms the new Spinulosa Clade with two additional new species, and H. fomiticola, which belongs to the Semiorbis Clade and forms effuse to large subpulvinate stromata on Fomes fomentarius, a trait new for species with green ascospores. Anamorph names are established prospectively in order to provide a basis for possible policy alterations towards their use for holomorphs.