Molds are a group of organisms that belong to the taxonomic kingdom of Fungi. There are over 20,000 species of mold. Molds reproduce by making spores. Mold spores waft through the indoor and outdoor air continually. When mold spores land on a damp spot indoors, they may begin growing and digesting whatever they are growing on. Molds can grow on virtually any organic substance, as long as moisture and oxygen are present.