Lager is a well attenuated beer brewed in cool conditions using a slow-acting brewers yeast, known as a bottom-fermenting yeast, and then stored (or "lagered") for a period in cool conditions to clear away particles and certain flavour compounds to produce a clean taste.

"Lager" yeast is a bottom-fermenting yeast, and typically undergoes primary fermentation at 7-12°C (45-55°F) (the "fermentation phase"), and then is given a long secondary fermentation at 0-4°C (30-40°F) (the "lagering phase"). During the secondary stage, the lager clears and mellows. The cooler conditions also inhibit the natural production of esters and other byproducts, resulting in a "crisper" or "cleaner" tasting beer.

After fermentation, bottom fermented beers are then stored in refrigerated cellars for lagering (maturing and clarification). Quality "Lagers" are stored at a low temperature (as low as 0°C/32°F) for several weeks to several months, clearing, acquiring mellowness, and becoming charged with carbon dioxide. With modern improved fermentation control and less discerning customers, commercial lagers only receive short periods of cold storage, typically 1–3 weeks.