The English Cocker Spaniel is a breed of gun dog. It is noteworthy for producing one of the most varied numbers of pups in a litter among all dog breeds. The English Cocker Spaniel is an active, good-natured, sporting dog standing well up at the withers and compactly built. There are "field" or "working" cockers and "house" cockers. It is one of several varieties of spaniel and is the foundation of its American cousin, the American Cocker Spaniel. The English Cocker is closer to the working-dog form of the Field Spaniel and the English Springer Spaniel. English Cocker Spaniels are also known as the "Merry Spaniel" due to their constantly wagging tails.

Outside the US, the breed is usually known simply as the Cocker Spaniel, as is the American Cocker Spaniel within the US. The OED states that the word cocker is derived from the fact the breed was used to flush woodcocks.

Spaniel type dogs have been found in art and literature for almost 500 years.^{[better source needed]} Initially, spaniels in England were divided among land spaniels and water spaniels. The differentiation among the spaniels that led to the breeds that we see today did not begin until the mid-19th century. During this time, the land spaniels became a bit more specialised and divisions among the types were made based upon weight. According to the 1840 Encyclopedia of Rural Sports, Cockers were 12–20 lb (5.5–9 kg).^{[better source needed]} At this time it was not uncommon for Cockers and Springers to come from the same litter. Even a puppy from a "Toy" sized lineage could grow to be a springer.^{[better source needed]}

There is no indication from these early sources that spaniels were used to retrieve game. Rather they were used to drive the game toward the guns.^{[better source needed]}

During the 1850s and 1860s, other types of Cockers were recorded. There were Welsh Springer Spaniels and Devonshire Cockers. Additionally, small dogs from Sussex Spaniel litters were called Cockers.^{[better source needed]} In 1874 the first stud books were published by the newly formed kennel club. Any spaniel under 25 lb (11 kg) was placed in the Cocker breeding pool, however, the Welsh Cocker was reclassified as a Springer in 1903 due to its larger size and shorter ear.^{[better source needed]}

The sport of conformation showing began in earnest among spaniels after the Spaniel Club was formed in 1885. When showing, the new Springer and Cocker, both were in the same class until The Spaniel Club created breed standards for each of the types. The Kennel Club separated the two types eight years later. Since then, the Springer and Cocker enthusiasts have bred in the separate traits that they desired. Today, the breed differs in more ways than weight alone.^{[citation needed]}

At Crufts, the English Cocker Spaniel has been the most successful breed in winning Best in Show, winning on a total of seven occasions between 1928 and 2009, with wins in 1930, 1931, 1938, 1939, 1948, 1950 and 1996. In addition, the breed makes up three of the four winners who have won the title on more than one occasion with all three coming from H. S. Lloyd's Ware kennel. Due to World War II, the English Cocker Spaniel managed to be the only breed to have won the title between 1938 and 1950, although the competition was only held on four occasions during that period. The most recent best in show was Sh. Ch. Canigou Cambrai in 1996.

In the late 19th century with the increase in popularity of dog shows and the creation of standards for various breeds, the Spaniel family began to split into various different breeds. A group of enthusiasts decided to create a large black spaniel breed. Four dogs would act as progenitors for this new breed, of which two were Cocker Spaniels, one was a Cocker Spaniel/English Water Spaniel cross and one was a Norfolk Spaniel. This new breed was named the Field Spaniel, and was recognised by the Kennel Club in 1892.^{[citation needed]}