The growth of hyphae results in discoloration and a blurred appearance, especially in foods. The network of these tubular branching hyphae, called mycelium, is considered a single organism. The hyphae are generally transparent, so the mycelium appears as very fine, fluffy white threads on the surface. Transverse walls (septums) may delimit connected compartments along the hyphae, each containing one or several genetically identical nuclei. The powdery texture of many molds is caused by the profuse production of asexual spores (conidia) formed by differentiation at the ends of hyphae. The mode of formation and shape of these spores is traditionally used to classify molds. Many of these spores are colored, making the fungus much more obvious to the human eye at this stage of its life cycle.[13]​[14]​.

Molds cause biodegradation of natural materials, which can be unwanted when it results in food spoilage or property damage. They also play an important role in biotechnology and food science in the production of various pigments, foods, beverages, antibiotics, pharmaceuticals and enzymes. Some animal and human diseases can be caused by certain molds: the disease can result from allergic sensitivity to mold spores, from the growth of pathogenic molds within the body, or from the effects of ingested or inhaled toxic compounds (mycotoxins) produced by molds.[15]​.

There are thousands of known species of molds with diverse lifestyles, including saprotrophs, mesophiles, psychrophiles and thermophiles, and some mycorrhizal, parasites of animals, fungi and plants, as well as opportunistic pathogens of humans. All require moisture to grow and some live in aquatic environments. Molds typically secrete hydrolytic enzymes, primarily from the tips of hyphae. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances that can be absorbed by hyphae. In this way, molds play an important role in the decomposition of organic matter, allowing the recycling of nutrients in all ecosystems. Many molds also synthesize mycotoxins and siderophores that, together with lytic enzymes, inhibit the growth of competing microorganisms. Molds can also grow on stored animal and human foods, making the food unpalatable or toxic and therefore a major source of food loss and illness. Many food preservation strategies (salting, pickling, jamming, bottling, freezing, drying) are to prevent or slow the growth of mold, as well as the growth of other microbes.[16]​.

Molds reproduce by producing a large number of small spores, which may contain a single nucleus or be multinucleated. Mold spores can be asexual (products of mitosis) or sexual (products of meiosis); many species can produce both types. Some molds produce small hydrophobic spores that are adapted for wind dispersal and can remain airborne for long periods; In some, the cell walls are darkly pigmented, providing resistance to ultraviolet radiation damage. Other mold spores have slimy sheaths and are more suitable for dispersion in water. Mold spores are often single spherical or ovoid cells, but can be multicellular and of various shapes. Spores can adhere to various surfaces, such as animal fur or plumage, which contributes to their dispersal; some are capable of surviving extreme temperatures and pressures.[17]​.

Although molds can grow on dead organic matter anywhere in nature, their presence is visible to the naked eye only when they form large colonies. A mold colony does not consist of discrete organisms, but is instead an interconnected network of hyphae called mycelium. All growth occurs at the tips of the hyphae, with cytoplasm and organelles flowing forward as the hyphae advance over or through new food sources. Nutrients are absorbed at the tip of the hypha. In artificial environments such as buildings, humidity and temperature are often stable enough to encourage the growth of mold colonies, commonly seen as a soft or hairy coating that grows on food or other surfaces.[16]​.

Few molds can begin to grow at temperatures of 4°C (39°F) or lower, so foods are usually refrigerated at this temperature. When conditions do not allow growth, molds can remain alive in a dormant state depending on the species, within a wide range of temperatures. Different mold species vary greatly in their tolerance to extreme temperatures and humidity. Certain molds can survive in harsh conditions, such as the snow-covered soils of Antarctica, refrigeration, highly acidic solvents, antibacterial soap, and even petroleum products such as jet fuel.[16]​.

Xeric molds can grow in relatively dry, salty or sugary environments, where the water activity is less than 0.85; Other molds need more humidity.[3]​.

The growth of hyphae results in discoloration and a blurred appearance, especially in foods. The network of these tubular branching hyphae, called mycelium, is considered a single organism. The hyphae are generally transparent, so the mycelium appears as very fine, fluffy white threads on the surface. Transverse walls (septums) may delimit connected compartments along the hyphae, each containing one or several genetically identical nuclei. The powdery texture of many molds is caused by the profuse production of asexual spores (conidia) formed by differentiation at the ends of hyphae. The mode of formation and shape of these spores is traditionally used to classify molds. Many of these spores are colored, making the fungus much more obvious to the human eye at this stage of its life cycle.[13]​[14]​.

Molds cause biodegradation of natural materials, which can be unwanted when it results in food spoilage or property damage. They also play an important role in biotechnology and food science in the production of various pigments, foods, beverages, antibiotics, pharmaceuticals and enzymes. Some animal and human diseases can be caused by certain molds: the disease can result from allergic sensitivity to mold spores, from the growth of pathogenic molds within the body, or from the effects of ingested or inhaled toxic compounds (mycotoxins) produced by molds.[15]​.

There are thousands of known species of molds with diverse lifestyles, including saprotrophs, mesophiles, psychrophiles and thermophiles, and some mycorrhizal, parasites of animals, fungi and plants, as well as opportunistic pathogens of humans. All require moisture to grow and some live in aquatic environments. Molds typically secrete hydrolytic enzymes, primarily from the tips of hyphae. These enzymes degrade complex biopolymers such as starch, cellulose and lignin into simpler substances that can be absorbed by hyphae. In this way, molds play an important role in the decomposition of organic matter, allowing the recycling of nutrients in all ecosystems. Many molds also synthesize mycotoxins and siderophores that, together with lytic enzymes, inhibit the growth of competing microorganisms. Molds can also grow on stored animal and human foods, making the food unpalatable or toxic and therefore a major source of food loss and illness. Many food preservation strategies (salting, pickling, jamming, bottling, freezing, drying) are to prevent or slow the growth of mold, as well as the growth of other microbes.[16]​.

Molds reproduce by producing a large number of small spores, which may contain a single nucleus or be multinucleated. Mold spores can be asexual (products of mitosis) or sexual (products of meiosis); many species can produce both types. Some molds produce small hydrophobic spores that are adapted for wind dispersal and can remain airborne for long periods; In some, the cell walls are darkly pigmented, providing resistance to ultraviolet radiation damage. Other mold spores have slimy sheaths and are more suitable for dispersion in water. Mold spores are often single spherical or ovoid cells, but can be multicellular and of various shapes. Spores can adhere to various surfaces, such as animal fur or plumage, which contributes to their dispersal; some are capable of surviving extreme temperatures and pressures.[17]​.

Although molds can grow on dead organic matter anywhere in nature, their presence is visible to the naked eye only when they form large colonies. A mold colony does not consist of discrete organisms, but is instead an interconnected network of hyphae called mycelium. All growth occurs at the tips of the hyphae, with cytoplasm and organelles flowing forward as the hyphae advance over or through new food sources. Nutrients are absorbed at the tip of the hypha. In artificial environments such as buildings, humidity and temperature are often stable enough to encourage the growth of mold colonies, commonly seen as a soft or hairy coating that grows on food or other surfaces.[16]​.

Few molds can begin to grow at temperatures of 4°C (39°F) or lower, so foods are usually refrigerated at this temperature. When conditions do not allow growth, molds can remain alive in a dormant state depending on the species, within a wide range of temperatures. Different mold species vary greatly in their tolerance to extreme temperatures and humidity. Certain molds can survive in harsh conditions, such as the snow-covered soils of Antarctica, refrigeration, highly acidic solvents, antibacterial soap, and even petroleum products such as jet fuel.[16]​.

Xeric molds can grow in relatively dry, salty or sugary environments, where the water activity is less than 0.85; Other molds need more humidity.[3]​.