Molds (or moulds; see spelling differences) are fungi that grow in the form of multicellular filaments called hyphae. A connected network of these tubular branching hyphae has multiple, genetically identical nuclei and is considered a single organism, referred to as a colony. In contrast, fungi that grow as single cells are called yeasts.

Molds are considered to be microbes and do not form a specific taxonomic or phylogenetic grouping, but can be found in the divisions Zygomycota, Deuteromycota and Ascomycota. Some molds cause disease or food spoilage; others play an important role in biodegradation or in the production of various foods, beverages, antibiotics and enzymes.