The kingdom Fungi constitutes an independent group equal in rank to that of plants and animals. It is a diverse clade of heterotrophic eukaryotic organisms that shares some ch- acteristics with animals and includes mushrooms, molds, yeasts as well as many other types of less well known organisms. Approximately 100,000 species have been described, which comprise less than 10% of the estimated number of fungal species in nature. Fungi can be found in every place wherever adequate moisture, temperature, and organic substrates are available; however, they also occupy extreme habitats, from hot volcanoes to arctic zones, arid deserts, and deep oceans. The importance of fungi as a group is tremendous; most species are saprobes and play prime roles in decomposition and the recycling of organic matter and nutrients, and many of them produce enzymes and metabolites with important applications in pharmacology, biotechnology, and other industries. Alongside the positive aspects, fungi also cause huge damage, primarily as plant pathogens. Fungi are highly amenable to molecular work, and a few fungal species serve as model systems to study basic processes with results that are applicable to many organisms, including humans.