Yeasts are best known for their beneficial contributions to society, and the literature abounds with discussions of their role in the fermentation of alcoholic beverages, bread, and other products. Yeasts also cause spoilage, but, with a few exceptions, this unwanted activity often goes unrecognized and underestimated as a major problem in the food and beverage industries. In some cases, there is only a fine line between what is perceived as either a spoilage or beneficial activity. This review examines the occurrence and growth of yeasts in foods and beverages with respect to their spoilage activities, the biochemistry of this spoilage, and technologies for the enumeration and identification of spoilage yeasts.