Reining is a western riding competition for horses where the riders guide the horses through a precise pattern of circles, spins, and stops. All work is done at the lope, or the gallop. Originating from working cattle, reining is often described as a Western form of dressage riding, as it requires the horse to be responsive and in tune with its rider, whose aids should not be easily seen, and judges the horse on its ability to perform a set pattern of movements. The horse should be willingly guided or controlled with little or no apparent resistance and dictated to completely. A horse that pins his ears, conveying a threat to his rider, refuses to go forward, runs sideways, bounces his rear, wrings his tail in irritation or displays an overall poor attitude is not being guided willingly, and is judged accordingly.

Western riding is considered a style of horse riding which has evolved from the ranching and welfare traditions which were brought to the Americas by the Spanish Conquistadors, as well as both equipment and riding style which evolved to meet the working needs of the cowboy in the American West. At the time, American cowboys had to work long hours in the saddle and often over rough terrain, sometimes having to rope a cattle using a lariat, also known as a lasso. Because of the necessity to control the horse with one hand and use a lariat with the other, western horses were trained to neck rein, that is, to change direction with light pressure of a rein against the horse's neck. Horses were also trained to exercise a certain degree of independence in using their natural instincts to follow the movements of a cow, thus a riding style developed that emphasized a deep, secure seat, and training methods encouraged a horse to be responsive on very light rein contact.