The Cavalier King Charles Spaniel is a small spaniel classed as a toy dog by The Kennel Club and the American Kennel Club. It originated in the United Kingdom and is one of the most popular breeds. Since 2000, it has grown in popularity in the United States and ranks as the 18th most popular pure-breed in the United States (2013 Registration Statistics). It has a silky, smooth coat and commonly a smooth undocked tail. The breed standard recognizes four colours: Blenheim (chestnut and white), Tricolor (black/white/tan), Black and Tan, and Ruby. The breed is generally friendly, affectionate and good with both children and other animals; however, they require a lot of human interaction. The expected average lifespan of a Cavalier King Charles Spaniel is under ten years.

The King Charles changed inordinately in the late 17th century, when it was interbred with flat-nosed breeds. Until the 1920s, the Cavalier shared the same history as the smaller King Charles Spaniel. Breeders attempted to recreate what they considered to be the original configuration of the breed, a dog resembling Charles II's King Charles Spaniel of the Restoration. Various health issues affect this particular breed.

The King Charles Spaniel (also known as the English Toy Spaniel) is a small dog breed of the spaniel type. In 1903, the Kennel Club combined four separate toy spaniel breeds under this single title. The other varieties merged into this breed were the Blenheim, Ruby and Prince Charles Spaniels, each of which contributed one of the four colours available in the breed.

Thought to have originated in the Far East, toy spaniels were first seen in Europe during the 16th century. They were made famous by their association with King Charles II of England (1630–1685) and have been linked with English royalty since the time of Queen Mary I (1516–1558). Members of the breed have been owned by Grand Duchess Anastasia Nikolaevna of Russia, Queen Victoria and Queen Elizabeth II.

The King Charles Spaniel and the other types of toy spaniels were crossbred with the Pug in the early 19th century to reduce the size of the nose, as was the style of the day. The 20th century saw attempts to restore lines of King Charles Spaniels to the breed of Charles II's time. These included the unsuccessful Toy Trawler Spaniel and the now popular Cavalier King Charles Spaniel. The Cavalier is slightly larger, with a flat head and a longer nose, while the King Charles is smaller, with a domed head and a flat face.