In 1994 the World Conservation Union (IUCN) published revised Red List categories, with criteria and guidelines on how they should be used. While this a substantial improvement on the previous system, it was clear that it had to be interpreted in different ways for different taxonomic groups. In particular, assigning threat categories to lesser-known groups such as cryptogams and invertebrates required the criteria to be interpreted in an appropriate way. This led the European Committee for the Conservation of Bryophytes (ECCB) to produce guidelines on interpreting the categories specifically for bryophytes. The numerical thresholds in the IUCN publication apparently require much quantitative data. Since these sorts of data are rare for bryophytes, evaluation against the threat categories must often be done by inference from what data are available. The most relevant data that can be used for bryophytes are population decline, present distribution and total population size, number of sites, and estimated loss of habitats over a specified period of time. It is thought that the guidelines developed for bryophytes may be applicable to a wide range of cryptogams, including lichens, perhaps with some modification. The revised IUCN categories have been applied with some success to lichens in Great Britain.