Fertilization is a process of the fusion of a sperm and an ovum. The process requires about 24 hours and with fertilixation embryonic life commences. Many critical events intervene between the formation of mature gametes and their final fusion in fertilization. The viability of oocytes and sperm is limited to couple of days in the female genital tract. Meeting of a spermatozoon and an ovum occur frequently in the ampulla of the uterine tube and seems to be encountered one another by mechanical means. Before a sperm can fertilize an oocyte, it must undergo a series of changes known as capacitation and acrosome reaction reaction in a sperm and denudation in an ovum. The sperm has to pass through the corona radiata to reach the surface of the zona and it appears to results from the enzymatic action of the tubal mucosa and possibly semen, probably movements of the tail of the sperm help it to penetrate the corona radiata and zona pellucida. After the sperm head attaches to the surface of the oocyte, two apparent changes occur to inhibit the entry of more sperms. The changes are the zonal reaction which appears in the zona pellucida and the cortical reaction which occur in cortical area of the oocyte. Once within the cytoplasm of the oocyte the head of the sperm enlarges to form male pronucleus and approach to the female pronucleus in the center of the oocyte where they come into contact and lose their nuclear membranes. Thus the maternal and paternal chromosomes intermingle at metaphase of the first mitotic division of the zygote. As a result of fertilization there appears four distinct phenomena : 1) restoration of diploid number of chromosomes, 2) variation of human species due to cross-over of chromosomes, 3) sex determination of the embryo, and 4) initiation of cleavage.