A genus composed of approximately 250 subdivided species representing the blue and green molds, the family of Penicillium molds includes such notable species as P. chrysogenum, from which the invaluable antibiotic drug penicillin is derived.

Other notable species are P. camemberti and P. roquefortii that are used to make popular cheeses, and many lesser known species, such as P. expansum and P. digitatum, which are known for their negative impacts on the fruit industry.

Molds are members of the Fungi kingdom, and are therefore neither plant nor animal. While many species of mold possess physical features strongly reminiscent of plants, such as stalk-like growths and plant-like cell walls, these resemblances are merely superficial. For example, the cell walls of plants are composed of cellulose, whereas a mold's cell wall is made of chitin. Furthermore, molds have no chlorophyll and are unable to produce their own food by means of photosynthesis. Rather, molds are heterotrophs and must obtain nutrients by consuming organic materials, a process achieved by secreting digestive enzymes that break down food materials into absorbable form.

These microscopic creatures have a very simple construction: most species grow in the form of hyphae, which are tubular, filamentous multicellular arrangements. A mass of hyphae is termed mycelium, and as an individual continues to feed and grow, new strands of hyphae are continuously formed and added to the collective group. In this way, a mold is able to branch out and envelop its food source.

The majority of mold species are capable of both asexual and sexual reproduction. Asexually, many species will grow stalk-like sporangiophores, which function to produce thousands of spores inside spore cases known as sporangia. Each spore, comprised of a nucleus and a protective coat, can withstand extremely harsh environmental conditions and is often transported by wind, water, or other organisms to be deposited a significant distance away. Given sufficiently warm, moist conditions, the spore will germinate and begin growing the first hypha. Alternatively, new mold growths are also able to arise from detached pieces of mycelium. Sexual reproduction, however, requires two complementary mating types that possess special reproductive side branches. Contact between these branches stimulates the production of gametes, which combine to form a zygote. A new zygote quickly develops a strong protective coating, similar to that of a spore, and is thereafter termed a zygospore. Zygospores are transported and deposited in the same manner as spores, where they too may eventually germinate and begin to grow hyphae.