Warmbloods are a group of middle-weight horse types and breeds primarily originating in Europe and registered with organizations that are characterized by open studbook policy, studbook selection, and the aim of breeding for equestrian sport. The term distinguishes these horses from both heavy draft horses ("cold bloods") and refined light saddle horses such as the Thoroughbred, Arabian, and Akhal-Teke ("hot bloods"). Although modern warmbloods are descended from heavier agricultural types systematically upgraded by hotblood influence, the term does not imply that warmbloods are direct crosses of "cold" and "hot".
Open studbook policies separate most warmbloods from true "breeds" such as Thoroughbreds, Arabians, Percherons, and Morgans which have a closed stud book and require two purebred parents. Instead, most warmblood registries accept breeding stock from other similar populations to continuously improve their own, and do not consider their own horses to be a discrete "breed". The Trakehner is an exception, as although some other breeds are used within the breeding population, this horse is considered a true breed. The Hanoverian, Holsteiner, and Selle Français studbooks are also considered slightly less open than others. Most warmblood registries recognize breeding stock from any other registry that is a member of the World Breeding Federation for Sport Horses which is affiliated with the IOC-recognized International Federation for Equestrian Sports.
A defining characteristic of a warmblood registry is studbook selection, though even some purebred breeds in Europe use this practice. Studbook selection is the use of external evaluation – critiquing conformation and movement – of potential breeding stock to cull unsuitable breeding horses and direct the evolution towards a particular goal. Today, studbook selection usually entails a performance proof in addition to external evaluation, particularly for stallions.
Standards of conformation and movement are not designed to perpetuate a particular ancestral type, but rather to meet a particular need. This concept is illustrated by the history of the Oldenburg horse through the past 150 years: in the late 19th century, the standard called for a heavy but elegant, high-stepping carriage horse, in the early 20th century for a heavier, stronger, economical farm and artillery horse, and since 1950 for a modern sport horse.
The most critical characteristic of a warmblood registry is that its breeding goal (or "breeding aim") is to breed sport horses. Each registry has a slightly different focus, but most breed primarily for show jumping and dressage. Many include combined driving and eventing as well. The breeding aim is reflective of the needs of the market. In eras and regions which called for cavalry mounts, warmbloods were bred to fit that need; when and where horses for light to moderate agricultural work were needed, warmbloods have also filled those roles. The purposeful evolution of the standard breeding aim is another characteristic of the warmbloods.
Warmbloods have become popular since the end of World War II when mechanization made agricultural horses obsolete, and recreational riding became more widespread in the western world. The ancestral warmblood types are referred to as the heavy warmbloods and are preserved through special organizations. The heavy warmbloods have found their niche as family horses and in combined driving.
Most warmbloods were developed in continental Europe, especially Germany. It was once thought that the warmblood type, which originated in continental Europe, descended from wild, native proto-warmblood ancestors, called the Forest Horse, though modern DNA studies of early horses have disproven this hypothesis.
The best-known German warmbloods are the Hanoverian, Holsteiner, Oldenburg and the purebred Trakehner. Others include the Württemberger, Rhinelander, Westphalian, Zweibrücker, Brandenburger, Mecklenburger, and Bavarian Warmblood. Several of these breeds are also represented by ancestral types such as the Ostfriesen and Alt-Oldenburger, Alt-Württemberger, and Rottaler.
Western European warmbloods include the French Selle Français, Belgian Warmblood, Dutch Warmblood, Swiss Warmblood, Austrian Warmblood and Danish Warmblood. Scandinavian countries also produce high-quality warmbloods such as the Finnish Warmblood and Swedish Warmblood.