Yeasts and moulds are terms to denote groups of fungi that commonly occur in various foods and beverages. Both groups can cause spoilage of foods, but they are also used in the production of some of our staple foods. Neither group forms a specific taxonomic or phylogenetic rank among the many thousands of fungi. Moulds include various fungal species that form microscopic filaments, called hyphae, whereas yeasts are mostly unicellular microorganisms. Large numbers of unicellular propagules (spores and conidia) may be produced at the tips of hyphae and may involve extension of cells or the production of large numbers of unicellular propagules. In contrast, single cells of yeasts multiply by budding or less often by fission. The mode of propagation can have great impact on the rate of growth and the method selected for monitoring cell counts or cell mass. Moreover, there are substantial physiological differences between moulds and yeasts. Moulds are, with few exceptions, strictly aerobic organisms, whereas the majority of yeasts occurring in foods are fermentative. Notwithstanding, all yeast species grow well under aerobic conditions. Furthermore, several mould species but not yeasts can produce mycotoxins in foods. Figures 23.1–23.4 illustrate mould colonies on several different media.