The St. Bernard Dog is a very large breed of dog, a working dog from the Swiss Alps, originally bred for rescue. The breed has become famous through tales of alpine rescues, as well as for its enormous size.

The Great Dane's large and imposing appearance belies its friendly nature; the breed is often referred to as a gentle giant. Great Danes are generally well-disposed toward other dogs, other non-canine pets and humans. Some individuals may chase or attack small animals, but this is not typical of the breed

The very fast growth rate and the weight of a St. Bernard can lead to very serious deterioration of the bones if the dog does not get proper food and exercise. Many dogs are affected by hip dysplasia or elbow dysplasia. Osteosarcoma (bone cancer) has been shown to be hereditary in the breed.

St. Bernards are susceptible to eye disorders called entropion and ectropion, in which the eyelid turns in or out. The breed standard indicates that this is a major fault.

The breed is also susceptible to epilepsy and seizures, a heart disease called dilated cardiomyopathy, and eczema.

Due to the likelihood of health problems in later years, the average lifespan for a Saint Bernard is around 7–10 years

The ancestors of the St. Bernard share a history with the Sennenhunds, also called Swiss Mountain Dogs or Swiss Cattle Dogs, the large farm dogs of the farmers and dairymen of the Swiss Alps, which were livestock guardians, herding dogs, and draft dogs as well as hunting dogs, search and rescue dogs and watchdogs. These dogs are thought to be descendants of molosser type dogs brought into the Alps by the ancient Romans, and the St. Bernard is recognized internationally today as one of the Molossoid breeds.

The earliest written records of the St. Bernard breed are from monks at the hospice at the Great St. Bernard Pass in 1707, with paintings and drawings of the dog dating even earlier.

The most famous St. Bernard to save people at the pass was Barry (sometimes spelled Berry), who reportedly saved somewhere between 40 and 100 lives. There is a monument to Barry in the Cimetière des Chiens, and his body was preserved in the Natural History Museum in Berne.

The classic Saint Bernard looked very different from the St. Bernard of today, because avalanches killed off many of the dogs used for breeding between 1816 and 1818. Severe weather during this period led to an increased number of avalanches that killed many St. Bernards while performing rescue work. In an attempt to preserve the breed, the remaining St. Bernards were crossed with Newfoundlands in the 1850s, and so lost much of their use as rescue dogs in the snowy climate of the alps because the long fur they inherited would freeze and weigh them down.

The Swiss St. Bernard Club was founded in Basel on March 15 1884. The St. Bernard was the very first breed entered into the Swiss Stud Book in 1884, and the breed standard was finally approved in 1888. "Since that time the St. Bernard has been a Swiss national dog."

Great Danes generally live 8–10 years, but with responsible breeding and improved nutrition they can live to be 12-14.

Dilated cardiomyopathy (DCM) and many congenital heart diseases are also commonly found in the Great Dane, leading to its nickname of the Heartbreak breed, in conjunction with its shorter lifespan. Great Danes also suffer from several genetic disorders that are specific to the breed. For example, if a Great Dane lacks color (is white) near its eyes or ears then that organ does not develop and usually the dog will be either blind, deaf, or both.[