A lichen flora of Northumberland is presented; it accepts 589 species. During the preparation of this work 7 lichens new to Britain and 271 new to the county were discovered. Complementing the species inventory is an analysis of factors affecting lichen distribution, a description of the major lichen habitats and a section on the history of lichen recording. Extensive tracts of semi-natural vegetation, steep climatic gradients and some of the least polluted air in England provide conditions suitable for a rich lichen flora; 40% of the British species have been recorded. Particularly notable habitats are sandstone crags in the uplands, Whin Sill (basalt) outcrops and sites contaminated with heavy metals. The County contains a major parkland, and several sites with a lichen flora characteristic of ancient woodland. A short list of lichenicolous fungi is included.