Beer mainly consists of water, it serves as a medium for all other substances in the beer.

The composition of the water affects the end result in several ways. The two best-known examples of this are the water from the Czech Plzen and the British Burton-upon-Trent. The very soft water (few minerals) of Plzen is perfect for brewing a smooth and clean beer like Pilsener. The hard, mineral-rich water from Burton-upon-Trent makes the hop bitters in (India) Pale Ales stand out more.

It is therefore mainly the minerals in the water that are important, it concerns the amount, composition and ratio of minerals. This also greatly influences the effectiveness of the enzymes during step 3 of the brewing process (mashing).

Brewers can adjust their brewing water and determine the hardness themselves. Burton-upon-Trent even got a verb from it: Burtonize.