Mastiffs are a group of large, solidly built breeds of dogs, typically with heavy bones, pendant ears, a relatively short and well muscled neck, and a short muzzle. The English Mastiff breed is also sometimes called simply a Mastiff. Although some mastiff breeds are used for search and rescue, such as the Saint Bernard and the Newfoundland, most are used as guard dogs, due to their deep voices and natural guarding instincts, or herding dogs, not for actual herding but for protection against large predators as well as poachers. Some breeds, like the Greater Swiss Mountain Dog have also been used as cart dogs.

The word "Mastiff" is derived from the same word as "massive", meaning "mountain-like". Related words are "molosser" (from Molossia , a country once located in what is now Western Greece), "dogge" (Germanic) and "douge" or "dogo" (romance languages).

The origin of the Mastiff is disputed, but Assyrian bas-relief carvings of Mastiffs found in Nineveh date back as far as approximately 640 BC. Many believe that the Tibetan Mastiff is the ancestor of modern Mastiffs, although there is little evidence to support this theory. It is a fact, though, that large watchdogs have existed in Asia and the Middle East for several thousand years.

Some of today's Mastiff breeds come from the British Isles and points farther north, which accounts for their thick coats and solid build. The name "Mastiff" is also used specifically for one breed, also known as the English Mastiff or Old English Mastiff.

The Bulldog breeds split from the Mastiffs in England and spread to the New World with colonization as well as Western Europe and, though smaller, are considered by some to still be mastiff breeds.

Mastiff breeds include the following: