Leptographium have been known since the early part of the 20th Century and include of many species causing blue stain of timber. Among these species are several species known or believed to be involved in causing diseases of trees. Leptographium spp. occur mainly on conifers and many species are recognized as anamorphs of Ophiostoma. Similar to Ophiostoma, Leptographium spp. are closely associated with insects. Their morphology thus reflects this association, and they thus have upright conidiophores with slimy masses that are produced in beetle galleries. Leptographium spp. are morphologically very similar to each other and this makes their accurate identification difficult. The first part of this thesis, presents dichotomous, as well as synoptic keys for the identification of these species. These keys are supported by comprehensive descriptions accompanied by both photographs and line drawings. The second part of this thesis. deals with several key taxonomic questions pertaining to Leptographium. Chapter one represents a phylogenetic study of the majority of species in Leptographium. Morphological characters were coded and analyzed. The results of the molecular and the morphological analyses are compared to determine whether any morphological characters might be used to infer phylogeny. The results indicate that morphology does not infer phylogenetic relatedness. Chapter two represents a comparison between Leptographium abietinum and L. engelmannii. These species are morphologically similar, and various authors have suggested that they are synonyms. Based on morphology, L. engelmanni was synonomised with L. abietinum. Furthermore, examination of various atypical isolates led to the description of the new species, L. hughesii. In chapter three, Ophiostoma europhioides, O. piceaperdum and Ceratocystis pseudoeurophioides are compared. These species have Leptographium anamorphs and are morphologically identical. Both O. europhioides and C. pseudoeurophioides are synonymised with O. piceaperdum, and a name is provided for the anamorph of O. piceaperdum. Chapter four represents a re-evaluation of Phialocepha/a phycomyces. The inconspicuous collarettes, characteristic of this fungus, are unlike the deep¬seated collarettes of the type species of Phialocepha/a (P. dimorphospora). Scanning and transmission electron microscopy revealed that conidiogenesis in P. phycomyces is phialidic, placing this species among other Phialocephala spp. However, P. phycomyces is able to tolerate high concentrations of cycloheximide, characteristic of Leptographium spp. DNA analysis indicates that this species does not belong in either Phialocepha/a or Leptographium. A new genus Kendrickiella is described to accommodate this species. In chapter five, a new species of Leptographium, L. eucalyptophilum, is described. This species is unique in that it occurs on Eucalyptus, which is an unusual host for this species. In addition, this species is one of several described from tropical regions and it is apparently adapted to this habitat. Chapter six represents a critical re-evaluation of isolates identified as L. procerum. Morphological comparison of these isolates revealed that L. procerum sensu lato, represents more than one taxon. From this study, three new species of Leptographium were described. These are L. alethinum, L. pityophilum and L. euphyes. These species can easily be distinguished from L. procerum s. str. and their incorrect identification is probably as a result of their shared habitat. In chapter seven, I describe an additional three species of Leptographium. Like most other Leptographium spp., these were isolated from conifers. The first of these, L. pineti, originates from Indonesia. The other two species is found in high elevation sites in Eastern North America. These are L. abicolens and L. peucophilum. These species are unique in that they are associated with the conifer swift moth, which is an unusual insect associate of Leptographium. Chapter eighth presents a description of a new species of Leptographium from Russia. This species, L. sibiricum, is associated with staining and mortality in siberian fir (Abies sibirica). The role of the fungus in the disease complex is still unknown, and awaits further study. This thesis represents a comprehensive review of all known, as well as newly described species. It should greatly facilitate plant pathologists and mycologists in the identification of Leptographium spp. This should lead to extensive pathogenicity tests, to determine the economic impact of species in this genus as blue-stain fungi and pathogens. It is my sincere wish that it will renew interest in this group of fungi, and will lead to the description of many more species in this genus.