Of all the herbs that have been used to flavor and preserve beer over the ages, only the hop (Humulus lupulus L.) is now regarded as an essential raw material in brewing throughout the world. Although hops were cultivated in Babylon as far back as 200 A.D., there is no record of their use to make beer until about 1079. The value of hops for flavor and preservation of alcoholic beverages appears to have been recognized by the twelfth century, and, in the thirteenth century, the hop started to threaten traditional herbs, such as rosemary, yarrow, coriander, and bog myrtle. The Bavarian Purity Law of 1516 decreed that only hops could be used for bittering beers in Germany. For a long time, sweet and strong ales were preferred in England, and the hop was condemned as a "wicked and pernicious weed." Good sense eventually prevailed, however, and hops were first grown in England in 1524, 100 years before the first gardens were established in North America. Although selection of improved varieties must have been a continuous process for most of the second millennium, the breeding of hops as an organized, scientific operation dates from the start of the twentieth century with a program initiated at Wye College, England, in 1904. In the same year, the α-acid humulone was first isolated as a crystalline solid, and during the next 60 years our knowledge of hop chemistry and understanding of the isomerization process was developed, leading to great benefits for breeders, growers, and brewers alike. Over the last 40 years, there has been a rapid proliferation of types of processed hops, or hop products, which offer the brewer several economic and quality advantages over the use of whole hops, and the contribution of hops to beer aroma, bitterness, foam, and light stability can now be controlled more effectively than ever before.