A fungus is any member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. The Fungi are classified as a kingdom that is separate from plants and animals. The discipline of biology devoted to the study of fungi is known as mycology or fungal biology, which is historically regarded as a branch of botany, even though genetic studies have shown that fungi are more closely related to animals than to plants. Fungi reproduce via spores and grow as hyphae, mycelia, and futher specialized structures. Fungal spores are often produced on specialized structures or in fruiting bodies, such as the head of a mushroom. Abundant worldwide, most fungi are mostly invisible to the naked eye because of the small size of their structures, and their cryptic lifestyles in soil, on dead matter, and as symbionts of plants, animals, or other fungi. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange. They have long been used as a direct source of food, such as mushrooms and truffles, as a leavening agent for bread, and in fermentation of various food products, such as wine, beer, and soy sauce.

Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological agents to control weeds and pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals including humans. The fruiting structures of a few species are consumed recreationally or in traditional ceremonies as a source of psychotropic compounds. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases or food spoilage can have a large impact on human food supplies and local economies. Despite their importance on human affairs, little is known of the true biodiversity of Kingdom Fungi, which has been estimated at around 1.5 million species, with about 5% of these having been formally classified.