The necessity for a well-reasoned and well-arranged system of classification is self-evident: without a working knowledge of the plants that are the subject of study no progress can be made. The recognition of plants as isolated individuals is not sufficient, it must be possible to place them in relation to others; hence the importance of a natural system. In identifying species artificial aids, such as habitat and substratum, are also often of great value, and a good working system should take account of all characteristics. Lichen development is the result of two organisms mutually affecting each other, but as the fungus provides the reproductive system, it is the dominant partner: the main lines of classification are necessarily determined by fruit characters. The algae occupy a subsidiary position, but they also are of importance in shaping the form and structure of the thallus. The different phyla are often determined by the presence of some particular alga; it is in the delimitation of families that the algal influence is of most effect. Zahlbruckner’s system gives due weight to the inheritance from both fungus and alga with, however, the fungus as the chief factor in development, and as his work is certain to be generally followed by modern lichenologists, it is the one of most immediate interest. His scheme has been accepted in the following more detailed account of families and genera, and for the benefit of home workers those that have not so far been recorded from the British Isles have been marked with an asterisk. It cannot be affirmed that nomenclature is as yet firmly established in lichenology. Both on historical grounds and on those of convenience, the subject is one of extreme importance, and interest in it is one of the main avenues by which we secure continuity with the past, and by which we are able to realize not only the difficulty, but the romance of pioneer work. Besides, there can be no exchange of opinion between students nor assured knowledge of plants, until the names given to them are beyond dispute. According to the ruling of the Brussels Botanical Congress in 1910, Linnaeus’s[1033] list of lichens in the _Species Plantarum_ has been selected as the basis of nomenclature, but since his day many new families, genera and species have been described and often insufficiently delimited. It is not easy to decide between priority, which appeals to the historical sense, and recent use which is the plea of convenience. Here also it seems there can be no rigid decision; the one aim should be to arrive at a conclusion satisfactory to all, and accepted by all.