Hops (Humulus lupulus) are a perennial climbing vine, a member of the family of Cannabinaceae and, hence, a close relative of Cannabis sativa. Only the female ripened flower is used for flavoring beer. There are presently over a hundred varieties of hops cultivated around the world.

Some of the best known are: Hallertauer, Fuggles, Tettnanger, East Kent Goldings, Styrian Goldings, Perle, Saaz, and Cascade, to name just a few. Apart from balancing the sweetness of the malt extract in beer through the contribution of bitterness, hops impart aroma and flavor, reduce the surface tension during the boiling stage, assist in forming a yeast head during ale fermentation and inhibit the growth of bacteria in wort and beer.