Thoroughbred breeding theories, or racehorse theories, are used by horse breeders in an attempt to arrange matings that produce progeny successful in horse racing. Bloodstock experts also rely on these theories when purchasing young horses or breeding stock. A basic understanding of these theories can also help the racing public understand a horse's theoretical genetic potential. The breeding theories stem from the belief that careful analysis of bloodlines can lend predictability to breeding outcomes. A well-designed mating increases the probability of the offspring's success, although many other factors also come into play.
Many thoroughbred breeding theories are implemented from other animal breeding stock practices, such as the use of inbreeding to "fix a type". Some breeding theories are qualitative, relying on judgement. Quantitative breeding theories usually focus on statistical analysis of the sire and broodmare sires in particular. The best-known classification system for mares was developed in the late 1800s by an Australian named Bruce Lowe, who analyzed the statistics of major race winners and ranked the distaff or mare lines by their degree of success. This and similar ranking systems are still used by some breeders today.