Bacteria have been recognized as important spoilage agents of beer since the end of the nineteenth century. Comparison of the descriptions of bacteria prevalent in breweries and beers at that time with those of today reveals that the range of bacteria encountered is remarkably constant, although the names have been changed frequently over the years. Recently, as new processes have been introduced, in particular the packaging of beers with very low oxygen contents, and microbiological techniques have been improved, so ‘new’ contaminants have been isolated from spoiled beers. These include the strictly anaerobic bacteria of the genera Pectinatus and Megasphaera (see Chapter 6), but in general brewers are facing today the same bacterial contaminants as they did in the nineteenth century.