This is the story of our ancient co-dependence with yeast, how microbe and man have led each other through history, and how the relationship is blossoming in the twenty-first century. From morning toast to evening wine, yeast is mankind’s daily bless-ing, and our communion with this little fungus is deepening year by year. It has taken us from hunting and gathering to more set-tled lives as farmers, provided our daily bread, and fed our thirst for wine and beer. Without yeast, earth would be an alcohol-free planet and every loaf of bread would be unleavened. In our time, yeast has become the darling of biotechnology, generating a cata-log of life-saving medicines, and billions of gallons of biofuel in the quest to slow climate change.