Penicillium is a genus of ascomycetous fungi of major importance in the natural environment as well as food and drug production. Members of the genus produce penicillin, a molecule that is used as an antibiotic, which kills or stops the growth of certain kinds of bacteria inside the body. According to the Dictionary of the Fungi (10th edition, 2008), the widespread genus contains over 300 species.
The genus was first described in the scientific literature by Johann Heinrich Friedrich Link in his 1809 work Observationes in ordines plantarum naturales. Link included three species—P. candidum, P. expansum, and P. glaucum—all of which produced a brush-like conidiophore (asexual fruiting structure). The common apple rot fungus P. expansum was selected as the type species.
In a 1979 monograph, John I. Pitt divided Penicillium into four subgenera based on conidiophore morphology and branching pattern: Aspergilloides, Biverticillium, Furcatum, and Penicillium.
Penicillium is classified as a genus of anamorphic fungi in the division Ascomycota (order Eurotiales, class Eurotiomycetes, family Trichocomaceae). The genus name is derived from the Latin root penicillium, meaning "painter's brush", and refers to the chains of conidia that resemble a broom.