Medical mycology is the study of fungi that are pathogens of humans and animals. Most fungi are filamentous, growing as tubelike structures or hyphae, but one important group are single celled and are referred to as the yeasts. The fungi are classified in a separate kingdom; they are eukaryotic like mammalian cells but different from bacteria, which are prokaryotic. Most fungi form spores asexually and reproduce sexually, although some lack a mechanism for sexual reproduction. They are very diverse in appearance microscopically. Therefore, the diagnostic laboratory technician is often confronted with problems in identification of fungal samples.