Fungi are ubiquitous, beneficial, harmful and mutualistic. They perform some of the most important basic roles in life and have some of the greatest potential for biotechnology, yet as few as 7% of the total estimated fungal species on Earth are described. There are thought to be 1.5 million fungal species, but there are huge problems in obtaining estimates of fungal diversity. These include: species recognition, as there are usually few useful characters to distinguish species; separate taxonomic binomials for asexual and sexual states of the same species; lack of specialist mycologists;
and the unfortunate downward trend for mycological biodiversity funding. Estimates of fungal diversity are discussed for selected plant groups, insects and species rich genera with more than 1,000 species. We conclude that it is important to identify habitats and substrates where a greater fungal diversity may occur in order to offer maximum protection to fungal resources. The large variation in estimates of fungal diversity means that considerable data are required before we can produce a reliable estimate of the number of species of fungi.