This chapter provides an excellent account of fungal diversity. Genera can be grouped into families, families into orders, orders into classes, and classes into phyla. Fungi studied by mycologists include organisms from three kingdoms, Chromista, Protozoa, and Fungi. Protozoa include cellular slime moulds and plasmodial slime moulds. Chromistas are oomycetes whereas fungi includes chytridiomycetes, zygomycetes, ascomycetes, basidiomycetes, and mitosporic fungi. Yeasts are fungi which occur predominantly or exclusively in a unicellular state which are abundant on leaves, fruits and in the nectaries of flowers. Some yeasts produce ascospores, basidiospores, and anamorphic, lacking a sexual phase. The great significance of yeasts for humans is their ability to grow readily with high metabolic rates in nutrient rich solutions, especially in sugary liquids. This has resulted in their importance in production or spoilage of a variety of foods and beverages. A lichen species is an intimate symbiotic association of a fungus nearly always an ascomycete, but sometimes a basidiomycete and a photosynthetic microorganism. Lichens can be loosely grouped into crustose, foliose and fruticose forms, and are found in both hot and cold deserts and dominate about 8% of the earth's surface. Many lichens are very sensitive to atmospheric pollutants, especially to sulfur dioxide.