A new checklist of the lichen-forming, lichenicolous and allied fungi occuring in the British Isles (including Ireland) is presented. The total number of species accepted is 1701, distributed through 294 genera. Of these species 1471 are lichen-forming, 183 lichenicolous, and 47 allied fungi. In addition to incorporating the results of investigations published since the last checklishts, the list embodies a great deal of original work by the authors and their collaborators; as a result a considerable number of changes in nomenclature are made here for the first time, including one new genus (Herteliana) and 56 new combinations. Names utilized in previous checklists are cross-referenced and relevant papers cited under generic heads.