Hops are the flower clusters of the vine-like hop plant, Humulus lupulus. They are of the nettle family and are closely related to cannabis… yep, marijuana. There are over 100 different varieties currently in commercial production for use in beer. Several varieties of hops are referred to as "noble hops", and have played a key role in the evolution of European beers. These include: Hallertau, Spalt, Saaz, and Tettnanger.

The first documented use of hops as a bittering agent in beer dates back to 1079, although there are records of hop cultivation from 300 years earlier. The Germans were the first to use hops in flavoring beer, but it took some time for it to catch on. Prior to hops, a combination of herbs, roots, and spices known as "gruit" was used to counter the inherent sweetness of beer's malt. Each brewer prepared his own gruit, and in some instances it would include poisonous or psychotropic ingredients such as henbane or deadly nightshade.

There is some debate as to what drove brewers to nearly uniformly adopt hops as the bittering agent of choice. Some credit the Protestant Reformation, with German princes disconnecting themselves from the Church, which controlled the production of gruit. Others cite a concern about public health, since gruit was not a standard recipe, and could include the poisonous herbs mentioned above. Still others cite hops's preservative qualities as being ideal for extending the shelf-life of beer.

In modern brewing, hops are one of the most commonly glorified ingredients of beer. Their use and popularity has created a subclass of beer afficianado known as a "hop head". These individuals seek out the hoppiest of beers; beers so bitter that they challenge the palate. One hop in particular, the Cascade hop, has garnered a huge following and is the trademark of at least two beer styles (the American Pale Ale and the new Cascadian Dark Ale, or Black IPA).

The oil or resin of the hop flower is what's most important in brewing. Depending on when hops are added in the brewing process, they can contribute to the bittering flavor or the aroma of the beer. Many beers require multiple hop additions to achieve the desired results. Common terms for referring to the flavors imparted by hops include: floral, piney, citrusy, lemony, grassy, and spicey.