It is well known that even men who have numerically abnormal semen analyses can impregnate their partners. Moreover, not infrequently, a man having a normal semen analysis will fall to fertilize eggs In Vitro. Therefore, the traditional semen analysis based on the World Health Organization (WHO) criteria is not always predictive of male fertility. The biological functions of sperm necessary to actually fertilize an egg are not reflected in a semen analysis, nor are any of the other tests of sperm function infallibly indicative of a man's fertility potential. In the actual fertilization process, the sperm must first have enough energy to penetrate the mass of cumulus cells surrounding the egg, bind on to the egg's outer surface and then burrow through the zona pelludica to reach the center of the cytoplasm to deposit its genetic material. Abnormalities in any of these areas may be present, so the inability to fertilize can occur at many different levels. Consequently, tests of male fertility such as the Hamster Egg Penetration Test lack validity and should be abandoned. The only positive proof of normal sperm function is a fairly recent history of fathering a child or fertilization of eggs In Vitro.