Many craft brewers at work today conducted their first amateur fermentations inside a carboy. A carboy is a large container made from different types of glass and plastic depending on the application, and used for storing and serving liquids including beer and yeast slurries. Five-gallon glass carboys were once commonly used to hold purified water and spring water for dispense by water coolers; these are now replaced by plastic vessels of a similar shape.

In brewing they are mainly used by home brewers as fermentation and aging vessels with either an airlock or bung in the top to prevent contamination by airborne microbes. Unlike plastic buckets sometimes used for homebrew fermentations, carboys are easy to make airtight. In a lab setting they are used for storing quantities of media, sterile water, and chemicals cleanly and safely. Some breweries also use them for propagation of special yeast and bacterial strains.

Carboys range in volume between 1 and 15 US gallons (4—60 liters), with the most common size being 5 gallons. Smaller sizes are sometimes simply called jugs while larger sizes are “demijohns” (except in England where a demijohn is 1 UK gallon). Their shape can vary but is generally flat bottomed with straight sides and high shoulders tapering to a narrow (1—3 inch) neck. Some demijohns have round or cone shaped bottoms and are supported by wicker or plastic webbing.