The revised IUCN Red List categories (1994) focus on any past and future reduction and on the current distribution and frequency of the taxa. The criteria and sub-criteria to categorize organisms are fully quantitative and require corresponding data. In order to apply these criteria appropriately for the establishment of the Red List of Switzerland's epiphytic lichens, an intensive inventory was made from 1995 to 1999. Lichen data were recorded in three different surveys: A representative survey, based on a stratified random sample of 826 permanent observation plots, guaranteed the comparability between the different taxa; the mapping in squares of 400 km² gathered as much information as possible on the rarer species; the third survey included all other floristic lichen data. A total of 656 lichen taxa was recorded. Based on the representative data, many inconspicuous species turned out to be common. The conservation categories based on current distribution and frequency have been determined by estimating the extent of occurrence, the area of occupancy and the number of locations according to IUCN criterion B. 38% of the epiphytic lichens would be given the status of "threatened" if based on current distribution and frequency alone. For the final Red List the taxa will be tested against all applicable IUCN criteria and sub-criteria.