Beer is an alcoholic beverage that is produced from malted barley, water, and hops. The classes of compounds are presented according to their chemical classification. Owing to the presence of two or more functional groups, this classification is sometimes arbitrary. This chapter discusses major groups such as alcohols, acids, esters, carbonyl compounds, bases, and sulfur compounds. In addition, it provides information on some miscellaneous components. The group of bases comprises various kinds of compounds such as aliphatic and aromatic amines, pyrroles, pyridines, and pyrazines. The volatile compounds contribute to the flavor and in particular to the odor of a product. A class of very important flavor-active compounds in beer are the esters. Sulfur compounds are generally very flavor-active constituents in beers. The formation of these vicinal diketones is influenced by various factors, but the final concentrations are predominantly dependent on the yeast activity at the end of the main fermentation.