The term Ascomycete may refer to an accepted name for a class of fungi under the phylum Ascomycota. Alternatively, it may be used to refer to the phylum Ascomycota itself. Invariably, when someone identifies a species as an "ascomycete", this is in reference to the presence of the ascus. In such organisms, the ascus is the recognizable sexual structure commonly contained in an ascocarp (or ascoma) shaped as a sac. Hence, the descriptive name "Sac Fungi".

Some species of Ascomycete may not always have a functional spore-bearing ascus. Those belonging to the group previously known as Deuteromycota, also known as Fungi Imperfectii, are asexual in nature and therefore have no need for a sac to hold spores. However, they may possess at least a morphological or physiological analog of the ascus. Such species, for example Penicilium from where the medicine penicillin is derived, are recognized as Ascomycetes. The phrase "ascomycetous fungi" is also used to describe these kinds of organisms.

Ascomycetes are the largest group of fungi. There are well over 60,000 species identified and they are found widespread all over the world. In every continent, from Asia, Europe, North and South America, Australia, Africa and even in Antarctica, Ascomycetes abound. Some species are known to be able to develop adaptations and symbiotic relationships with various plants and animals (as in lichens) for survival in extreme environments. Ascomycete spores are ever-present in the atmosphere.

While some Ascomycete species can be found just about everywhere, there are others that are extremely localized. For example, Tuber magnatum (more popularly known as white truffle) is endemic only to specific areas of France and Italy. These species of habitat-limited fungi are mostly Ascomycetes that are, in most likelihood, symbionts with particular plant or animal life.

Phylum Ascomycota is further classified into 3 sub-phyla - Pezizomycotina, Saccharomycotina, and Taphrinomycotina. Because of the vast diversity of species under the Ascomycota group, they all come in multi-varied colors, smells, growth rates, shapes and sizes. Some Ascomycetes can be simple single-celled organisms such as Saccharomyces cerevisiae, commonly known as baker's yeast. The more complex multi-cellular Ascomycetes include Erysiphales (powdery mildew), Morchella (morels), Tuber (truffles), Cladonia (moss-like lichens), and Penicillium (penicillin).

Several Ascomycetes are known pathogens to both plants and animals, including humans. Some of the plant diseases produced by Ascomycetes are Dutch elm disease, chestnut blight, powdery mildew, and ergot. The latter also affects humans by long-term ergot poisoning called ergotism. Other human diseases caused by Ascomycete species include candidiasis from Candida albicans, various dermatomycoses or skin infections from Epidermophyton, and pulmonary infections which may be severe and life-threatening for patients with impaired immune systems.

Despite the threats posed by various Ascomycetes, they serve a pivotal role in the ecology. They help in the decomposition of dead organic materials and are a source of food for insects, gastropods and even small to mid-sized mammals like a variety of rodents, pigs and deer.

Ascomycetes are also sources for food and medicine for humans. A notable example is Penicillin, which is derived from Penicillium chrysogenum. Other Ascomycetes are used for the production of insulin and HGH (human growth hormone) for therapeutic purposes.