Yeasts used in brewing and winemaking (e.g. S. cerevisiae) are unicellular fungi that mainly belong to the Ascomycetes (sac fungi) and Deuteromycetes (imperfect fungi) classes. They do not possess chlorophyll, cannot photosynthesize (Madigan et al., 2003), and therefore utilize complex foods for their nutrients, which they can metabolize with or without oxygen present. Each yeast cell possesses one membrane bound nucleus enclosing the chromosomes (in contrast to other fungi which may be multinuclear). The genome of S. cerevisiae has been under scrutiny for decades, with no less than 12 genetic maps of the organism published between 1960 and 1997 (Boulton and Quain, 2001a). It is probably reasonable to claim that S. cerevisiae are genetically the best understood of all organisms, and this intimate knowledge of the genome has many applications in fermentation technology. As fungi, S. cerevisiae are classified as ascomycetes because they produce ascospores through meiosis (sexual reproduction). This is triggered when yeast are nutritionally stressed, for example by deprivation of a carbon or nitrogen source. The diploid yeast in the population produces four haploid nuclei, which are incorporated into four stress-resistant ascospores, encapsulated inthe ascus. Another characteristic of S. cerevisiae is that they are also able to grow vegetatively, i.e. they can reproduce asexually by budding multilaterally or extruding a bud from a point on the mother cell (Jackson, 2000a). When this extrusion reaches about half the size of the parent cell it is pinched off and becomes a daughter cell, leaving a ‘budscar’ on the mother cell. Buds may arise at any point on the mother cell surface, but never again at the same site. Branched chaining (pseudohyphae) may occasionally follow multilateral budding when buds fail to separate. Under optimal nutritional and cultural conditions S. cerevisiae double their mass every 90 minutes (Pretorius, 2000). Vegetative reproduction is normal under the conditions found in must and wine, as ascal development is suppressed by high concentrations of glucose, ethanol or carbon dioxide.