Yeasts are single cells, which reproduce by budding. Some species form of a string of connected budding cells known as pseudohyphae. If sexual reproduction occurs, cell fusion occurs, followed by budding. Yeasts do not form a single taxonomic or phylogenetic grouping. The most common pathogens are Candida species, and they fall in the Ascomycota. Also important are the basidiomycete yeasts, notably Cryptococcus species. There are probably hundreds of thousands species of yeasts; here we profile the most important human pathogens.