Yeasts can be defined as those fungi whose asexual growth predominantly results from budding or fission, and which don’t form their sexual states within or upon a fruiting body. The evident ubiquity of yeasts in the Earth’s biosphere is balanced by their diversity, specificity for habitats, and relationships with other organisms. They are a versatile group of eukaryotic microorganisms exhibiting heterogeneous nutritional profiles and a surprising ability to survive in a wide range of natural and altered habitats and to colonize a variety of ecosystems within a wide range of climates, altitudes, substrates and geographical locations. In addition, yeasts are probably one of the most relevant microbial groups in biotechnology. Mankind have benefited from yeasts for millennia in traditional fermentation technologies.