Dog fighting is a type of blood sport that turns game and fighting dogs against each other in a physical fight, often to the death, for the purposes of gambling or entertainment to the spectators. In rural areas, fights are often staged in barns or outdoor pits; in urban areas, fights are often staged in garages, basements, warehouses, alleyways, abandoned buildings, neighborhood playgrounds, or in the streets. Dog fights usually last until one dog is declared a winner, which occurs when one dog fails to scratch, dies, or jumps out of the pit. Sometimes dog fights end without declaring a winner; for instance, the dog's owner may call off the fight.

Dog fighting generates revenue from stud fees, admission fees, and gambling. Most countries have banned dog fighting, but it is still legal in some countries, such as Venezuela, Bangladesh, Japan and Albania. The sport is also popular in Russia.^{[needs update]}

Blood sports in general can be traced back to the Roman Empire. In 13 BC, for instance, the ancient Roman circus slew 600 African beasts. Dog fighting, more specifically, can also be traced to ancient Roman times. In AD 43, for example, dogs fought alongside the Romans and the British in the Roman Conquest of Britain. In this war, the Romans used a breed that originated from Greece called the Molossus; the Britons used broad-mouthed Mastiffs, which were thought to descend from the Molossus bloodline and which also originated from Greece. Though they lost this war, the Romans were so impressed with the Britons' warhounds that they began to import these dogs for use in the Colosseum, as well as for use in times of war. While spectators watched, the imported dogs were pitted against animals such as elephants, lions, bears and bulls, and also against gladiators.

Later, the Romans bred and exported fighting dogs to Spain, France and other parts of Europe until eventually these dogs made their way back to England. Though bull-baiting and bear-baiting were popular throughout the Middle Ages up to the 19th century in Germany, France, Spain, Portugal, and the Netherlands, the British pitted dogs against bulls and bears. In 12th century England during the feudal era, the landed aristocracy, who held direct military control in decentralized feudal systems and thus owned the animals necessary for waging war, introduced bull baiting and bear baiting to the rest of the British population. In later years, bull-baiting and bear-baiting became a popular source of entertainment for the British royalty. For instance, Queen Elizabeth I, who reigned from 1558 to 1603, was an avid follower of bull- and bear-baiting; she bred Mastiffs for baiting and would entertain foreign guests with a fight whenever they visited England. In addition to breeding Mastiffs and entertaining foreign guests with a fight, Queen Elizabeth, and later her successor, King James I, built a number of bear gardens in London. The garden buildings were round and roofless, and housed not only bears, but also bulls and other wild animals that could be used in a fight. Today, a person can visit the Bear Garden Museum near the Shakespeare Global Complex in Bankside, Southwark.^{[citation needed]}

With the popularity of bull- and bear-baiting, bears needed for such fights soon became scarce. With the scarcity of the bear population, the price of bears rose and, because of this, bull-baiting became more common in England over time. Bulls who survived the fights were slaughtered afterwards for their meat, as it was believed that the fight caused bull meat to become more tender. In fact, if a bull was offered for sale in the market without having been baited the previous day, butchers were liable to face substantial fines. Animal fights were temporarily suspended in England when Oliver Cromwell seized power, but were reinstated again after the Restoration. Dog fighting, bull-baiting, and bear-baiting were officially outlawed in England by the Cruelty to Animals Act 1835. The official ban on all fights, however, actually served to promote dog fighting in England. Since a small amount of space was required for the pit where a dog fight took place, as compared to the ring needed for bull- or bear-baiting, authorities had a difficult time enforcing the ban on dog fighting.

In 1817, the bull and terrier crossbreeds were brought to America and dog fighting slowly became part of American culture. Yet, though historical accounts of dog fighting in America can be dated back to the 1750s, it was not until the end of the Civil War (1861–1865) that widespread interest and participation in the blood sport began in the United States. For instance, in 1881, the Mississippi and Ohio railroads advertised special fares to a dog fight in Louisville; public forums such as Kit Burns' Tavern, "The Sportman's Hall", in Manhattan regularly hosted matches. Many of these dogs thrown into the "professional pits" that flourished during the 1860s came from England and Ireland — where citizens had turned to dogs when bull-baiting and bear-baiting became illegal in their countries.

In 20th century America, despite the expansion of laws to outlaw dog fighting, dog fighting continued to flourish underground. Aiding in the expansion of dog fighting were the police and firemen, who saw dog fighting as a form of entertainment amongst their ranks. In fact, the Police Gazette served as a go-to source for information about where one could attend a fight. When Henry Bergh, who started the American Society for the Prevention of Cruelty to Animals (ASPCA), witnessed police involvement in these fights, he was motivated to seek and receive authority for the ASPCA Humane Law Enforcement Agents to have arresting power in New York. Additionally, Bergh's 1867 revision to New York's animal cruelty law made all forms of animal fighting illegal. However, According to the ASPCA website, the Humane Law Enforcement department of ASPCA has been disbanded and NYPD has taken over its duty. As laws were passed to outlaw the activity, high-profile organizations, such as the United Kennel Club, who once endorsed the sport by formulating rules and sanctioning referees, withdrew their endorsement.

On July 8, 2009, one of the largest dog fighting raids in U.S. history occurred. Law enforcement seized over 350 dogs, mostly pit bulls, and arrested 26 people across eight states. Most of the dogs were expected to have to be euthanized, as their harsh upbringing did not prepare them to be able to be safely placed in an adoptive home.