The taxonomy of lichenized fungi (lichens) is relatively well-known compared to that of other fungi because lichens are long-lived and possess a thallus that requires exposure to light. Therefore they are easily observed on their natural substratum and sampling can be carried out much more thoroughly than in other fungal groups. Comprehensive catalogues of lichen taxa known worldwide are available and currently about 13500 species are accepted.

An analysis of selected, recently revised groups suggests that there has been a mean increase in species number of about 25% since 1931. One third of the species recognized before 1931 were withdrawn, and almost half of the currently recognized species have been described after 1931. Some 3000 names seem to be ‘orphaned’, not included in currently accepted genera and not appearing in recent species counts. New species are being described at an increasing rate, so that large numbers of ‘missing lichens’ seem to be present. However, there is no clear indication how numerous they are, only an estimation that they amount to some 25% of all species.

A geographical analysis of checklists suggests that most missing species will be found in the tropics and the southern hemisphere. A taxonomic analysis of recently published species suggests that there is no particular group yielding more novelties than others. Our own fieldwork impressions suggest that tropical primary forests are the most productive in revealing undescribed species. These are found in particular on leaves and on bark high up on the tree trunks. In well-studied areas like western Europe the new discoveries tend to be cryptic species or species rarely producing ascocarps.

In conclusion, the ‘missing’ lichens are estimated at about 4000, and are to be found everywhere and in all taxonomic groups, but predominantly in primary tropical forests. This would result in a total of about 18000 lichen species.