In British forestry sweet chestnut (Castanea saliva Mill.) is considered an interesting proposition for the future as it is predicted to respond better to global warming and related climate change than native hardwood species. Information about the ectomycorrhizal (ECM) fungal community associated with C. sativa is scant in comparison to traditional timber species of the northern hemisphere. As these fungi are essential to the growth and development of their host, improved knowledge about the ECM fungal associates of C. sativa will be valuable. Using ECM root tip morphotyping (morphological identification), molecular analysis (internal transcribed spacer region amplification and sequencing), and fruitbody sampling, this thesis considers diversity and composition of the ECM fungal community associated with C. sativa, inside and outside its natural distribution range (NOR), in two southern European countries (Italy and Portugal) and two British countries (Wales and England), respectively. The ECM fungal root tip communities in the six investigated C. sativa stands varied largely in composition and structure. Overall, 95 unique ECM fungal types were identified on 704 77 analysed root tips, none of which where shared between all study sites. With a presence at 5 sites the ECM fungus Cenococcum geophilum had the widest distribution. C. sativa roots sampled in Italy and Portugal were predominantly colonised by C. geophilum. Roots sampled at the British C. sativa stands were for the largest part colonised by members of the Russulaceae family. Many species of the latter family also dominated the sporocarp communities in the British C. sativa stands. [n the latter stands 1306 sporocarp specimen were observed over three fruiting seasons that could be assigned to 43 fungal taxa. Differences in ECM fungal root tip community composition were particularly outspoken between the sites located inside and outside the NOR of C. sativa. Yet, community similarity between the southern European sites as well as between the sites located in the UK was low too. The latter dissimilarity is partly ascribed to the fact that four of the investigated C. sativa stands suffered the effects of different forms of stand management (a sylvicultural thinning treatment or soil tillage) and natural disturbance, (Phytophthora Ink Disease). These anthropogenic and biotic impacts generally resulted in shifts in the species composition of the ECM fungal communities. Only the soil tillage regime appeared to have a negative impact on overall ECM fungal biodiversity.