Equine reproduction is an evolving field with the use of artificial insemination and possibly embryo transfer. Regardless of the method used, the primary objective is the selection of the sire on his fertility. The best evaluation of a stallion's fertility is the measurement of fertility per cycle, but when this measurement is not available, other estimators exist such as, testicular measurements, testicular weight, hormonal levels, and sexual behavior assessment.The estimation of the sexual function of the stallion is difficult due to the variable means of investigation. Therefore, we will discuss here the subject of stallion reproduction, and more specifically the methods to evaluate the characteristics of the sexual function.