Dry-aged beef is beef that has been hung or placed on a rack to dry for several weeks. After the animal is slaughtered and cleaned, it is hung as a full or half carcass. Primal (large distinct sections) or sub primal cuts, such as strip loins, rib eyes, and sirloin, are placed in a refrigerator unit, also known as a "hot box". This process involves considerable expense, as the beef must be stored near freezing temperatures. Subprimal cuts can be dry aged on racks either in specially climate-controlled coolers or within a moisture-permeable drybag. Moreover, only the higher grades of meat can be dry aged, as the process requires meat with a large, evenly distributed fat content. Because of this, dry-aged beef is seldom available outside of steak restaurants and upscale butcher shops or groceries. The key effect of dry aging is the concentration and saturation of the natural flavour, as well as the tenderization of the meat texture.
The process changes beef by two means. Firstly, moisture is evaporated from the muscle. The resulting process of desiccation creates a greater concentration of beef flavour and taste. Secondly, the beef's natural enzymes break down the connective tissue in the muscle, which leads to more tender beef.
The process of dry-aging usually also promotes growth of certain fungal (mold) species on the external surface of the meat. This does not cause spoilage, but rather forms an external "crust" on the meat's surface, which is trimmed off when the meat is prepared for cooking. These fungal species complement the natural enzymes in the beef by helping to tenderize and increase the flavor of the meat. The genus Thamnidium, in particular, is known to produce collagenolytic enzymes which greatly contribute to the tenderness and flavor of dry-aged meat. 
Dry-aged beef is typically not sold by most supermarkets in the U.S. today, because it takes time, the meat loses weight, and there is a risk of spoilage. Dry-aging can take from 15 to 28 days, and typically up to a third or more of the weight is lost as moisture. This type of beef is served in higher-priced steakhouses and by select restaurants.
Dry-aging can be done at home under refrigeration by three means: open air, with the presence of salt blocks, and with the use of a moisture permeable drybag to protect the meat while it is aging. Since the mid-2010s, some chefs have experimented with a "quick" or "cheat" dry-age by coating a cut of beef with ground koji (rice inoculated with Aspergillus oryzae) to simulate the effect of traditional dry-aging; the results are not quite the same, but can be achieved within 48 to 72 hours. The koji technique can also be applied to chicken and shrimp.
When dry aging using a moisture permeable material, surface mold growth is not present, flavor and scent exchange within the refrigerated environment is not a concern, and trim loss of the outer hardened surface is measurably reduced. The flavor and texture profile of the beef is similar on all dimensions to the traditional open air dry-aged results.
Historically, it was common to store mutton or beef joints at room temperature for extended periods; even after the invention of refrigeration hanging sides of beef in large coolers for a few weeks as part of the processing was standard.