This thesis represents a critical taxonomical review of the fungal genus Cryphonectria sensu lato. An appropriate taxonomy for this group is of great importance because it includes many well known tree pathogens such as the chestnut blight fungus Cryphonectria parasitica and the Eucalyptus canker pathogen Cryphonectria cubensis. The many taxonomic changes introduced in studies presented in this thesis have largely arisen as a result of DNA sequence comparisons for Cryphonectria spp. that show that Cryphonectria sensu lato is comprised of different lineages, strongly supported by robust morphological characteristics. New taxa, of which many are pathogenic, have also been discovered. The expanded number of species of Cryphonectria and related genera as well as the consideration of large numbers of isolates has furthermore made it possible to establish a broad view of the group at the super-generic level. The first part of the thesis deals with studies on Cryphonectria cubensis. A new genus Chrysoporthe is described for C. cubensis sensu lato. Two additional species are also described for phylogenetic sub-clades previously known as C. cubensis. These include Chrysoporthe austroafricana, representing all isolates from South Africa, and an anamorphic species described in the new genus Chrysoporthella as Chrysop. hodgesiana, which is currently only known from Colombia on native Tibouchina spp. Isolate collections from several new host genera for Chr. cubensis are also characterized. Collections from Eucalyptus in Cuba, now representing the epitype of Chr. cubensis, also define the type of Chr. cubensis as residing in the South American sub-clade. Another new species, Chrysoporthe inopima from Tibouchina lepidota in Colombia is described as well as a new species Chrysoporthe doradensis for isolates from Eucalyptus spp. in Ecuador. A new family Cryphonectriaceae is described in this thesis for Cryphonectria, Chrysoporthe and Endothia. Genera in this family are united by orange stromatic tissue, with the pigments colouring purple in 3% KOH and yellow in lactic acid. The existence of this new family confirms the close relationship of Cryphonectria and morphologically similar genera. A proposal to conserve the name Cryphonectria against the new type C. parasitica is presented. This is required because Cryphonectria gyrosa, the currently accepted type, was erroneously used as type. The conservation of Cryphonectria against C. parasitica made it possible to describe the new genus Amphilogia for C. gyrosa. Amphilogia also includes a second species from New Zealand described as Amphilogia major, although no isolates currently exist for this species. New genera for existing Cryphonectria spp., as well as newly discovered fungi are presented in this thesis. The new genus Rostraureum is established for a fungus pathogenic on Terminalia ivorensis in Ecuador. This fungus also represents a new species, Rostraureum tropicale. Cryphonectria longirostris, originating from Puerto Rico, Trinidad and Tabago, is also transferred to Rostraureum. A fungus morphologically similar to Chrysoporthe on native Tibouchina, Miconia and exotic Eucalyptus spp. in Colombia, is described as Aurapex penicillata gen. sp. nov. Cryphonectria havanensis is transferred to the new genus Microthia. Cryphonectria coccolobae also resides in this genus based on morphology, although its phylogenetic relationship to C. havanensis could not be confirmed due the absence of isolates. A new fungus was discovered during surveys for C. coccolobae on Coccoloba uvifera in Florida, which is described in the new genus Ursicollum as U. fallax. Phylogenetic analyses in this study also clearly distinguish Cryphonectria eucalypti from Cryphonectria, and this fungus is thus transferred to the new genus Holocryphia. A minireview is presented at the end of the thesis and discusses the new taxonomic concepts developed for Cryphonectria during this thesis, and recent studies by other authors. The review describes how this new taxonomic scheme has changed our view and understanding of the distribution and ecology of Cryphonectria sensu stricto from what it has traditionally been seen. The final part of the thesis is written in the form of a monograph. It contains background information of all the species, including many pathogens, currently known in Cryphonectria and allied genera. The majority of these have recently been described, some in this thesis, and this chapter thus contains all recent information pertaining to them. It is intended that this monograph should be useful as a manual, enabling users to work with and isolate these fungi and to identify the different taxa based on morphology and phylogenetic relationships. The studies presented in this thesis greatly change the taxonomy of Cryphonectria sensu lato, which is now seen as representing a large number of genera and species in a new family. Many would argue that Cryphonectria is still monophyletic, but the different lineages shown by DNA sequence comparisons are morphologically inordinately diverse, and clearly represent different genera. Studies presented in this thesis further suggest that additional genera await description from diverse geographical areas and ecological niches. The studies presented in this thesis will hopefully provide a foundation against which these new taxa can be compared and will improve our understanding of tree diseases.