Horse racing is an equestrian sporting activity which has been practiced over the centuries; the chariot races of Roman times were an early example, as was the contest of the steeds of the god Odin and the giant Hrungnir in Norse mythology. Horse Racing is often inextricably associated with the activity of wagering on the outcome of a race, gambling .

The principal form of horse racing, which is popular in many parts of the world, is thoroughbred racing . Harness racing is somewhat popular in the United States, Canada and Scandinavia, and to a lesser extent elsewhere. Quarter horse racing is also popular in the United States.

The breeding, training and racing of horses in many countries is now a significant economic activity as, to a greater extent, is the gambling industry which is largely supported by it. In horse racing, exceptional horses can win millions of dollars and make millions more by providing stud services, such as horse breeding .

The style of horse racing, the distances and the type of events varies very much by the country in which the race is occurring, and many countries offer different types of horse racing.

In the United Kingdom for example, there is horse racing which involves obstacles (either hurdles or fences) called National Hunt horse racing and those which are unobstructed races over a given distance (flat racing). Horse racing in the UK has provided many of the sport's greatest ever jockeys, most notably Gordon Richards.

In the United States, horse racing can occur on flat surfaces of either dirt or grass, generally thoroughbred racing; other tracks offer quarterhorse racing and harness racing, or combinations of these three types of racing. Horse racing with other breeds, such as Arabian horse racing, is found on a limited basis. American thoroughbred races are run at a wide variety of distances, most commonly from 4.5 furlongs (905 m) to 1½ miles (2414 m); with this in mind, breeders of thoroughbred race horses are able to breed horses to excel at a particular distance (see Dosage Index).

The high point of US horse racing has traditionally been the Kentucky Derby which, together with the Preakness Stakes and the Belmont Stake, form the Triple Crown for three-year-olds. However, in recent years the Breeders' Cup races, held at the end of the year, have been challenging the Triple Crown events, held early in the year, as determiners of the three-year-old champion. They also have an important effect on the selection of other annual champions.

The corresponding standardbred horse racing event is the Breeders' Crown. There are also a Triple Crown of Harness Racing for Pacers and a Triple Crown of Harness Racing for Trotters .

American betting on horse racing is sanctioned and regulated by state governments, almost always through legalized parimutuel gambling.

Thoroughbred horse racing in the United States has its own Hall of Fame for horses, jockeys, and trainers.

While the attention of horse racing fans and the media is focused almost exclusively on the horse's performance on the racetrack or for male horses or possibly its success as a sire, but little publicity is given the brood mares. Such is the case of La Troienne, one of the most important mares of the 20th century to whom many of the greatest thoroughbred champions, and dams of champions can be traced.

In Australia horse racing the most famous horse is Phar Lap. However, this horse is from New Zealand, as was Cardigan Bay, a pacing horse who enjoyed great success at the highest levels of American harness racing in the 1960s. Racing in Australasia has enjoyed great success with races such as the Melbourne Cup, which has recently been attracting many international entries.

The most famous horses from Canada are Northern Dancer, who after winning the horse racing Kentucky Derby and Preakness went on to become the most successful thoroughbred sire ever, and his son Nijinsky II . In Canada, however, harness racing is more popular than Thoroughbred racing. Woodbine Racetrack in Toronto, home of the Queen's Plate, Canada's premier horse racing stakes race, and the North America Cup, Canada's premier standardbred stakes race, is the only race track in North America which stages Thoroughbred and Standardbred (harness) meetings on the same day. The North America Cup has the largest purse of any Canadian horse race.

In Ireland horse racing, noted for its great horse racing history, the Derby winning Thoroughbred race horse Shergar was kidnapped on February 8, 1983. He has never been found.

During horse racing some jockeys use a crop to hit the horse in the rump to direct their effort and possibly run faster. Based on the most detailed statistical study of whipping ever carried out, critics state that it is actually counterproductive and slows the horse down; becoming distracted and dangerously veer off course. In a sport that is not only highly competitive but relies on high-stakes gambling to sustain itself, some people feel it is unlikely that a practice shown to be a disadvantage would continue. In Norway horse racing, use of the whip is banned, except for in emergencies. It is also heavily restricted in India horse racing, where jockeys are not allowed to use a whip until they have proven they are capable of winning without it. The British Jockey Club responds to criticism of the whip by acknowledging that 'the Jockey Club has long advised riders that more races are lost rather than won through use of the whip'.

Some 375 in-racing horses die every year in the UK alone. They may die on the course itself, from injuries received in training or be killed by their owners - considered no longer commercially viable. The UK industry breeds three times more horses than enter horse racing. Many of these horses are used in breeding, sold to hunters who ride horseback, for point-to-pointing (another horse sport), and general recreational riding. Animal Aid, a UK animal rights organization, contends that some of these 'lost' horses are killed to be fed to hunting hounds, used for pet food, or exported for human consumption.