Since the publication of the first edition in 1978, in which 693 species were described, there have been considerable advances in our knowledge of British and Irish mosses and in the taxonomy of critical groups such as Andreaea and the Hypnum cupressiforme, Racomitrium heterostichum, R. canescens and Schistidium apocarpum complexes. Since 1978, eight species new to science have been described in Britain, 25 species not previously known in the British Isles have been discovered and taxonomic revisions have led to the addition of a further 51 species. Fourteen species have been reduced to varieties of or to synonymy with other species or removed from the British list. This has resulted in an increase of 70 in the number of species making a total of 763.

Additionally, critical studies of generic limits, as in the Amblystegiaceae and Pottiaceae, and DNA studies have led to a considerable increase in the number of genera – from 175 to 214 in the British Isles. There have also been major changes in the overall classification of mosses. Clearly, the first edition of this book was in dire need of updating. Apart from additions and taxonomic changes, the original descriptions of species have been revised and in some instances augmented and footnotes added or increased where thought necessary. The key to genera and keys to species have been revised in the light of experience since they were first published.