Traditionally, the division Bryophyta has included three classes, Hepaticopsida or Marchantiopsida (liverworts), Anthocerotopsida (hornworts) and Musci (mosses). Recent evidence, however, suggests that at an early stage in the evolution of terrestrial plants the liverworts diverged from other land plants, then hornworts diverged and finally mosses (see Willis & McElwain, 2002). Thus, the original concept of the division Bryophyta as consisting of three classes cannot be maintained and it is here considered to consist only of the mosses.

Until the beginning of the 1980s the classification of mosses was based upon the morphology and anatomy of the gametophyte and sporophyte and it was sometimes a matter of dispute as to which of the two should be used for a particular group. Over the past 20 years or so new techniques have been developed, especially DNA sequencing, which provide information on the genetic relationships of taxa. This has confirmed some older classifications but others have been shown to be incorrect. The classification of mosses is now in a state of flux as a result of the development of these new techniques, Rather than using a traditional classification, such as that used by Corley et al. (1981), which is clearly very out of date, I have followed the tentative classification of D. H. Vitt (in Shaw & Goffnet, 2000). However, it is likely that further studies will lead to refinements, and even major changes, to this classification.