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Beer is a complex alcoholic beverage made from barley (malt), hops, water and yeast. It is a good source of phenolic antioxidants. The majority of phenolic constituents of beer are derived from malt (70–80%), whereas about 20–30% are derived from hops. Phenolic constituents of beer represent a large structural variety and belong to the classes of simple phenols, benzoicand cinnamic acid derivatives, coumarins, catechins, di-, triand oligomeric proanthocyanidins, (prenylated) chalcones and fl avonoids. The structural diversity of these phenolic components that have been identifi ed and quantifi ed in beer will be summarized. Introduction Beer is one of the most consumed alcoholic beverages. Beer is rich in nutrients including carbohydrates, amino acids, minerals, and vitamins, but also provides a variety of nonnutrient components including phenolic compounds ( Piendl, 1989 ). In Germany, beer is brewed according to the Beer Purity Law from 1516 only from barley, hops and water, with the addition of yeast. Beer brewing is a complex process that allows variation in multiple parameters during brewing which infl uence the type and quality of beer. These include the variety of barley and the malting process, temperature and pH during mashing, sparging, boiling, the variety of hops added during wort boiling, and yeast fermentation. The phenol content of beer was determined as 500– 1,000 mg/l ( Leupold and Drawert, 1981 ). About 70–80% of beer polyphenols are malt-derived ( Knorr, 1978 ; De Keukeleire, 2000 ). Barley polyphenols undergo changes during the malting and brewing process and are less well characterized than phenolic compounds from hops ( Moll et al. , 1984 ). Hops ( Humulus lupulus L.) is added to beer in small quantities for aroma and bitterness and to provide antifungal and antibiotic properties ( Mizobuchi and Sato, 1985 ; Verzele, 1986 ; Stavri et al. , 2004 ). These characteristics are mainly attributed to their content of -acids like humulone, which are converted during brewing to the bitter-tasting iso-acids. However, the dried hops cones also contain a large quantity of polyphenols (4–14%) which account for about 20–30% of the polyphenols found in beer ( Knorr, 1978 ; Stevens et al. , 1998 ; De Keukeleire et al. , 1999 ; Taylor et al. , 2003 ). Description of Structural Classes of Phenolic Beer Constituents Phenolic constituents of beer cover a large structural variety and belong to the classes of simple phenols, benzoicand cinnamic acid derivatives, coumarins, catechins, di-, triand oligomeric proanthocyanidins, (prenylated) chalcones and fl avonoids. This chapter will summarize the structural diversity of these components that have been identifi ed and quantifi ed in beer. Each class of phenolic beer constituents will be described, mean quantities measured in beer sample will be summarized, and chemical structures of the beer constituents will be given. A detailed description of biological activities will be found elsewhere in this handbook. Simple phenols Simple volatile phenols in beer contribute to the typical beer aroma. They either have an unpleasant “ phenolic ” smell, or are characterized by a pleasant vanillin, clove-like spicy or smoky odor. Levels of simple phenols in beer are generally low ( Table 12.1 ). The aromatic alcohol tyrosol ( 5 ), which is formed by fermentation of the amino acid tyrosine ( Tressl et al. , 1976 ), was detected in exceptionally high concentrations up to 40 mg/l. Levels in alcohol-free beers were considerably lower than in standard beers (2.78  3.12 mg/l vs. 11.83 3.12 mg/l). This was attributed to differences in the duration of fermentation and yeast strains employed in brewing alcohol-free beers ( Bartolomé et al. , 2000 ). Interestingly, tyrosol ( 5 ) was identifi ed also as a major phenolic constituent of olive oil, with levels in the range of Phenolic Compounds in Beer Clarissa Gerhäuser German Cancer Research Center, Heidelberg, Germany Hans Becker Pharmacognosy and Analytical Phytochemistry, University of Saarland, Saarbrücken, Germany Copyright © 2009 Elsevier Inc. All rights of reproduction in any form reserved Beer in Health and Disease Prevention ISBN: 978-0-12-373891-2 12 Author’s personal copy Table 12.1 Summary of simple phenols and their contents in beer Simple phenols (mg/l) 1 4-Vinyl phenol OH CH2 Up to 0.15 a,b 2 4-Vinyl guiacol CH2 OH OCH3 Up to 0.55 a,b ; 0.68 c