Cheddar cheese is a relatively hard yellow to off-white, and sometimes sharp-tasting cheese originally made in the English village of Cheddar, in Somerset. Cheddar cheese is the most popular cheese in the United Kingdom, accounting for 51% of the country's £1.9 billion annual cheese market.

Cheddar cheese has been widely imitated, both in the United Kingdom and in other countries, including Ireland, the United States, Australia, New Zealand, South Africa, Canada and Iceland. In most countries 'Cheddar' cheese, in its various forms, is readily available, ranging from mild lower-fat cheeses to the more mature higher-fat and sharper cheeses. By European trading standards, however, only cheese produced and sourced in four of the counties of South West of England (Somerset, Devon, Dorset, and Cornwall) may be given the Protected Designation of Origin name "West Country Farmhouse Cheddar".

Cheddar cheese has been produced since at least 1170. A pipe roll of King Henry II from that year records the purchase of 10,420 lb at a farthing per pound (£3 per ton). One suggestion is that Romans brought the recipe to England from the Cantal region of France, where it was adapted. Cheddar cheese traditionally had to be made within 30 miles (48 km) of Wells Cathedral.

Central to the modernisation and standardisation of Cheddar cheese was the nineteenth century Somerset dairyman Joseph Harding. For his technical developments, promotion of dairy hygiene and unremunerated propagation of modern cheese-making techniques he has been described as the father of Cheddar cheese. Harding introduced new equipment into the process of cheese making, including his "revolving breaker" for curd cutting, saving much manual effort. The "Joseph Harding method" was the first modern system for Cheddar production based upon scientific principles. Harding stated that Cheddar cheese is 'not made in the field, nor in the byre, nor even in the cow, it is made in the dairy', He and his wife were behind the introduction of the cheese into Scotland and North America. Joseph Harding's son, Henry Harding, was responsible for introducing Cheddar cheese production to Australia.

During the Second World War most milk in Britain was used for the making of one single kind of cheese nicknamed 'Government Cheddar' as part of war economies and rationing. This nearly resulted in wiping out all other cheese production in the country. Before the First World War there were more than 3,500 cheese producers in Britain, while fewer than 100 remained after the Second World War.

In July 2009, the West Country Farmhouse Cheesemakers launched a wedge of Cheddar to the edge of space; although GPS contact was lost with the space vehicle (a balloon) the wedge was later recovered in good condition.

Cheddaring refers to an additional step in the production of Cheddar-style cheese where, after heating, the curd is kneaded with salt, then is cut into cubes to drain the whey, then stacked and turned. Strong, extra-mature Cheddar, sometimes called vintage, needs to be matured for up to 15 months. The cheese is kept at a constant temperature often requiring special facilities. As with production of other hard cheese varieties in other regions worldwide, caves provide an ideal environment for maturing cheese; still, today, some Cheddar cheese is matured in the caves at Wookey Hole and Cheddar Gorge.

The curds and whey are separated using rennet, an enzyme complex normally produced from the stomachs of new-born calves (in vegetarian cheeses bacterial, yeast or mould derived chymosin is used).

Cheddar style cheeses are produced in the United Kingdom, Ireland, Canada, United States of America, South Africa, New Zealand, Australia (where processed cheese varieties are often called tasty cheese), Sweden, Belgium, and the Netherlands. These varieties of cheese can be split in two categories, artisanal cheese and industrial cheese. Artisan varieties of Cheddar develop diverse and strong flavour over time. Industrial cheese's flavour varies significantly enough that food packaging will usually indicate a strength using adjectives such as mild, medium, strong, tasty, sharp, extra sharp, mature, old, or vintage, which may indicate the maturation period, or food additives used to enhance the flavour.

The name 'Cheddar cheese' has become widely used internationally, and does not currently have a protected designation of origin (PDO). However, the European Union recognises West Country Farmhouse Cheddar as a PDO. To meet this standard the cheese must be made in the traditional manner, using local ingredients, in one of four designated West Country counties: Somerset, Devon, Dorset, or Cornwall.

The Slow Food Movement, encouraged and advised by Neal's Yard Dairy, has recently created a Cheddar Presidium , claiming that only three cheeses should be called 'Cheddar'. Their specifications, which go further than the West Country Farmhouse Cheddar PDO, require that Cheddar cheese be made in Somerset and with traditional methods, such as using raw milk, traditional animal rennet, and a cloth wrapping.

Notable Cheddar cheeses from Somerset include "Quickes", which in 2009 was awarded cheese of the year by the British Cheese Association, "Keen's", with a strong tang, "Montgomery's", with an apple after taste and the unpasteurised Cheddar made by the Gorge Cheese Company in Cheddar itself, and a very fine example of a non Cheddar cheese, made in the style of a traditional Cheddar in Lincolnshire, UK, is "Lincolnshire Poacher".

The ideal quality of the original Somerset Cheddar was described by Joseph Harding in 1864 as "close and firm in texture, yet mellow in character or quality; it is rich with a tendency to melt in the mouth, the flavour full and fine, approaching to that of a hazelnut".

Cheddar, made in the classical way, tends to have a sharp, pungent flavour, often slightly earthy. Its texture is firm, with farmhouse traditional Cheddar being slightly crumbly, it should also, if mature, contain large crystals of calcium lactate - often precipitated when matured for times longer than 6 months. Real Cheddar is never 'soapy', in texture or mouth-feel.

Cheddar is usually a deep to pale yellow (off-white) colour, but food colourings are sometimes used in industrial varieties of Cheddar style cheeses. One commonly used example is Annatto, extracted from the tropical achiote tree. The largest producer of industrial Cheddar style cheese in the United States, Kraft, uses a combination of annatto and oleoresin paprika, an oil made from paprika.

Coloured Cheddar style cheese has long been on sale, but even as early as 1860 the real reason for this was unclear: Joseph Harding stated "to the cheese consumers of London who prefer an adulterated food to that which is pure I have to announce an improvement in the annatto with which they compel the cheesemakers to colour the cheese". According to David Feldman, an author of trivia books, "The only reason why cheesemakers colour their product is because consumers seem to prefer it.".

Cheddar cheese was sometimes (and still can be found) packaged in black wax, but was more commonly packaged in larded cloth, which was impermeable to contaminants but still allowed the cheese to "breathe", although this practice is now limited to artisan cheese makers.

In the United States of America, industrial and processed Cheddar-style cheeses come in several varieties, including mild, medium, sharp, extra sharp, New York Style, Colby/Longhorn, white, and Vermont. New York style Cheddar cheese is a particularly sharp Cheddar, sometimes with a hint of smoke. It is usually slightly softer than milder Cheddar varieties. Colby/Longhorn Cheddar cheese has a mild to medium flavour. The curds are still distinct, often marbled in colour, varying from cream to yellow. Cheddar that has not been coloured is frequently labelled as "white Cheddar" or "Vermont Cheddar", regardless of whether it was produced in the state of Vermont.

Many cheeses bear the Cheddar name, but are actually flavoured processed cheeses, and often bear little resemblance to the original cheese. Examples include Easy Cheese, a cheese contained in a spray can, or individually wrapped cheese slices.

Cheddar cheese is one of several products used by the United States Department of Agriculture to track the dairy industry; reports are issued weekly detailing prices and production quantities. The state of Wisconsin produces the most Cheddar cheese in the United States; other centres of production include California, Upstate New York, Vermont, and Tillamook, Oregon.

White House historians assert that U.S. President Andrew Jackson held an open house party where a 1,400 lb (635 kg) block of Cheddar cheese was served as 'refreshment'. This served as direct inspiration for "Big Block of Cheese Day", which was featured in two episodes of the Emmy-award winning television series The West Wing.

A cheese of 7,000 lb (3,175 kg) was produced in Ingersoll, Ontario, in 1866 and exhibited in New York and Britain; it was immortalised in the poem "Ode on the Mammoth Cheese Weighing over 7,000 Pounds" by James McIntyre, a Canadian poet.

In 1893 farmers from the town of Perth, Ontario produced The Mammoth Cheese, at a weight of 22,000 lb (10,000 kg) for that year's Chicago World's Fair. When placed on exhibit with the Canadian display, The Mammoth Cheese promptly crashed through the floor and had to be placed on reinforced concrete in the Agricultural Building. It was more written about than any other single exhibit at the fair, and received the bronze medal.

A still larger Wisconsin Cheddar cheese of 34,951 lb (15,853 kg) was produced for the 1964 New York World's Fair. It required the equivalent of the daily milk production of 16,000 cows.

See also this hilarious video about cheddar .