Summary The ascomycete H ymenoscyphus pseudoalbidus (anamorph C halara fraxinea ) causes a lethal disease known as ash dieback on F raxinus excelsior and F raxinus angustifolia in E urope. The pathogen was probably introduced from E ast A sia and the disease emerged in P oland in the early 1990s; the subsequent epidemic is spreading to the entire native distribution range of the host trees. This pathogen profile represents a comprehensive review of the state of research from the discovery of the pathogen and points out knowledge gaps and research needs. Taxonomy Members of the genus H ymenoscyphus ( H elotiales, L eotiomycetidae, L eotiomycetes, A scomycota) are small discomycetes which form their ascomata on dead plant material. A phylogeny based on the internal transcribed spacers ( ITS s) of the rDNA indicated the avirulent H ymenoscyphus albidus , a species native to E urope, as the closest relative of H . pseudoalbidus . Symptoms H ymenoscyphus pseudoalbidus causes necrotic lesions on leaves, twigs and stems, eventually leading to wilting and dieback of girdled shoots. Bark lesions are characterized by a typical dark‐ to cinnamon‐brown discoloration. Life cycle H ymenoscyphus pseudoalbidus is heterothallic and reproduces sexually on ash petioles in the litter once a year. Ascospores are wind dispersed and infect ash leaves during the summer. The asexual spores only serve as spermatia. Tools and techniques The most important techniques for fungal handling, such as detection, isolation, culturing, storage, crossing and ascocarp production, are briefly described. Management Once the disease is established, management is hardly possible. The occurrence of a small fraction of partially tolerant trees constitutes hope for resistance breeding in the future. Healthy‐looking trees should be preserved.