The causes of infertility can be divided into four major categories: 1) the female factor; 2) the male factor; 3) combined factors; 4) unexplained infertility. It is difficult to assign exact percentage to each of these categories; however, it is generally reported that in approximately 35% of cases, infertility is mainly due to a female factor, in 30% to a male factor, in 20% to abnormalities detected in both partners, and in 15% of cases no diagnosis can be made after a complete investigation. In some couples there is no possibility of natural conception because of sterility of the male (azoospermia or lack of ejaculation) or of the female (ovarian failure, tubal occlusion, absence of the uterus). Minor degrees of fertility impairment are not necessarily associated with couple infertility when present in only one partner but may reduce the couple’s fertility when present in both partners.