Ingredients

The basic ingredients of beer are water; a fermentable starch source, such as malted barley; and yeast. It is common for a flavouring to be added, the most popular being hops. A mixture of starch sources may be used, with the secondary starch source, such as corn, rice and sugar, often being termed an adjunct, especially when used as a lower cost substitute for malted barley.

Water
Beer is composed mostly of water, and water used to make beer nearly always comes from a local source. The mineral components of water are important to beer because minerals in the water influence the character of beer made from it. Different regions have water with different mineral components. As a result, different regions are better suited to making certain types of beer. For example, Dublin has hard water well-suited to making stout, such as Guinness, and Pilzen has soft water well-suited to making pale lager, such as Pilsner Urquell. As a result, it is argued that the mineral components of water have an influence on the character of regional beers.

Starch source
The starch source in a beer provides the fermentable material in a beer and is a key determinant of the character of the beer. The most common starch source used in beer is malted grain. Grain is malted by soaking it in water, allowing it to begin germination, and then drying the partially germinated grain in a kiln. Malting grain produces enzymes that convert starches in the grain into fermentable sugars. Different roasting times and temperatures are used to produce different colours of malt from the same grain. Darker malts will produce darker beers.
Nearly all beer includes barley malt as the majority of the starch. This is because of its fibrous husk, which is important in the sparging stage of brewing, and high concentration of amylase, a digestive enzyme which facilitates conversion of starch into sugars. Other malted and unmalted grains (including wheat, rice, oats, and rye, and less frequently, corn and sorghum) may be used.

Hops

Crushed hops
The flower of the hop vine is used as a flavouring and preservative agent in nearly all beer made today. The flowers themselves are often called "hops." The use of hops in beer was recorded by captive Jews in Babylon around 400 BCE. Hops were used by monastery breweries, such as Corvey in Westphalia, Germany, from 822 CE,though the date normally given for widespread cultivation of hops for use in beer is the thirteenth century.Hops contain several characteristics that brewers desire in beer: hops contribute a bitterness that balances the sweetness of the malt; hops also contribute floral, citrus, and herbal aromas and flavours to beer; hops have an antibiotic effect that favours the activity of brewer's yeast over less desirable microorganisms; and the use of hops aids in "head retention", the length of time that a foamy head created by carbonation will last. The bitterness of beers is measured on the International Bitterness Units scale. Beer is the sole major commercial use of hops.
In the past, other plants have been used for similar purposes; for instance, Glechoma hederacea. Combinations of various aromatic herbs, berries, and even ingredients like wormwood would be combined into a mixture known as gruit and used as hops are now used.

Yeast

Yeast is the microorganism that is responsible for fermentation in beer. Yeast metabolizes the sugars extracted from grains, which produces alcohol and carbon dioxide, and thereby turns wort into beer. In addition to fermenting the beer, yeast influences the character and flavour. The dominant types of yeast used to make beer are ale yeast (Saccharomyces cerevisiae) and lager yeast (Saccharomyces uvarum); their use distinguishes ale and lager. Brettanomyces ferments lambics, and Torulaspora delbrueckii ferments Bavarian weissbier. Before the role of yeast in fermentation was understood, fermentation involved wild or airborne yeasts. A few styles such as lambics rely on this method today, but most modern fermentation adds pure yeast cultures directly to wort.

Clarifying agent
Some brewers add one or more clarifying agents to beer. Common examples of these include isinglass finings, obtained from swimbladders of fish; kappa carrageenan, derived from seaweed; Irish moss, a type of red algae; polyclar (artificial), and gelatin. Clarifying agents typically precipitate out of the beer along with protein solids, and are found only in trace amounts in the finished product.