The taxa of this large moss genus are notorious for their morphological variability, causing many identification problems. In this work 93 species and 360 valid and legitimate synonyms are recognised. For each species, literature concerning the protologue, herbarium housing the type specimen, synonyms, geographical distribution in terms of the Index Muscorum, characters, and references are given. All recognised species are illustrated and for the majority of the taxa, colour photographs have been included on the CD-ROM.