This dissertation represents a study on Ceratocystis spp. with hat-shaped ascospores. Previously only six species of Ceratocystis with this spore form were known. These include C. fimbriata, C. pirilliformis, C. albofundus, C. moniliformis, C. moniliformopsis and C. acericola. In this study, we have discovered and tentatively described three new species with hat-shaped ascospores. One of these group with the larger C. fimbriata clade, while the other two reside within the larger C. coerulescens clade. Chapter one provides a concise summary of the literature pertaining to the genus Ceratocystis. The intention of the chapter is to introduce readers to this important genus of plant pathogens and to provide a background regarding their taxonomy, ecology, biochemistry and variation in species. Emphasis is placed on the distinction of Ceratocystis from Ophiostoma, as ell as on those Ceratocystis spp. with hat-shaped ascospores that negatively impact upon plantation forestry species. This chapter shows how Ceratocystis spp. associated with hardwood species in commercial forestry plantations have increased in number and it provides the background for research presented in the following five chapters. In the second chapter of this dissertation, a new species of Ceratocystis was discovered amongst isolates from the Himalayan mountain range of Bhutan. This fungus, in association with the bark beetle Ips schmutzenhoferi, is responsible for large-scale deaths of Himalayan spruce trees in Bhutan. The fungus is morphologically very similar to C. moniliformis and C. moniliformopsis, but differences in culture morphology, survival at different incubation temperatures and DNA sequence data based on three different gene regions supported the fact that this is a unique species. The fungus has thus been tentatively described as Ceratocystis bhutanensis prov. nom. In chapter 3, I consider isolates of a Ceratocystis sp. recently discovered associated with dying clove trees in Northern Sulawesi, Indonesia. The fungus was found at a very high level of incidence, but was at first identified as C. fimbriata, based on morphological characteristics. Differences were observed in cultures of this fungus when they were compared with C. fimbriata especially in terms of colony colour and growth at different temperatures. Morphological differences were also observed when the clove fungus was compared with C. fimbriata isolates. When three different DNA gene regions were sequenced and compared, it was clear that this fungus represents a new species. The fungus is, therefore, tentatively described as Ceratocystis polychrome prov. nom. in this dissertation. Ceratocystis polychrome prov. nom. Isolates obtained from cloves in Sulawesi displayed three distinctly different culture morphologies. In Chapter 4 of this dissertation we used DNA sequence data and microsatellite markers to consider whether these differences could be observed at the molecular level. Comparisons of sequence data for the ITS region gave no distinction between any of the morphological groups. A total of 50 isolates were studies using microsatellites markers developed for C. fimbriata. No distinction could be obtained between isolates representing the three different culture morphological groups. The 50 isolates were subsequently treated as one population in further analyses. With the aid of the microsatellite markers, it was shown that this population probably originated from Sulawesi and that it benefits form sexual outcrossing. In chapter five of this dissertation, a study was undertaken to consider the taxonomic status of C. moniliformis. Consideratble variation has been noted in different descriptions of this species. It also has a very wide host and geographic distribution raising speculation that C. moniliformis represents a species complex rather than a single taxon. Based on morphological and DNA sequence data from three gene regions, isolated from Sumatra were described as a new species, which we have tentatively named C. tribiliformis prov. nom. The other C. moniliformis isolates were all the same, despite the fact that they originated from a wide range of hosts and areas. The fungus correctly bearing the name C. moniliformis, C. moniliformis sensu stricto, therefore does not seem to tepresent a species complex. Species such as the closely related C. tribiliformis prov. nom., C bhutanensis prov. nom., C. omanensis prov. nom. and C. moniliformopsis all belong to the larger C. moniliformis sensu lato group, and all have hat-shaped ascospores, conical spines on the ascomatal bases, disc-shaped bases to the ascomatal necks and are phylogenetically closely related to C. moniliformis. Studies presented in this dissertation provide considerable new knowledge regarding various Ceratocystis spp. with hat-shaped ascospores. Three new species are described and I have also been able to show that C. moniliformis sensu stricto is monophyletic. Two of the species (C. tribiliformis prov. nom. and C. bhutanensis nom. prov.) group within the larger C. coerulescens clade while C. polychrome prov. nom. groups within the larger C. fimbriata clade. Studies in this dissertation have also improved our knowledge of the identity of several species previously incorrectly identified as either C. moniliformis or C. fimbriata. What has clearly emerged from this dissertation is the need for a monograph of Ceratocystis to include all new species and to thoroughly consider the population biology and ecology of all species.