The process whereby a spermatozoon and an oocyte fuse to form a single‐celled zygote is termed fertilisation. Following penetration of the vitelline membrane by the spermatozoon, the activated oocyte completes meiosis and extrudes the second polar body. The chromosomes contained in the haploid male pronucleus align with their corresponding chromosomes in the female pronucleus. The paternal and maternal chromosomes condense, become attached to mitotic spindles and align themselves centrally. The first mitotic, or cleavage, division follows. The integration of the paternal and maternal genetic material, which occurs during these processes, is referred to as syngamy. As a consequence of fertilisation, the diploid number of chromosomes is restored, the sex of the individual is determined (by the spermatozoon) and biological variation results from the integration of paternal and maternal hereditary characteristics.

Many aquatic animals release oocytes and spermatozoa into the water and fertilisation takes place in this aqueous environment. The fact that gametes are released at approximately the same time and in close proximity to each other, usually as a consequence of courtship, contributes to the likelihood of fertilisation taking place. In addition, mutual chemical attraction between male and female gametes increases the probability of fertilisation. This selective attraction is considered to be important in the attachment of the spermatozoon to the oocyte and in the inhibition of cross‐fertilisation between unrelated species. Relative to mammalian species, aquatic animals and amphibians produce large numbers of zygotes; however, the parental energy invested per zygote is low. In contrast, birds and mammals produce far fewer zygotes; thus, the parental energy invested per zygote is much greater. This investment can take many forms and in mammalian species it encompasses the parental investment provided both during gestation and postnatally.

In mammals, oocytes are retained within the female reproductive tract and are fertilised by spermatozoa deposited there. This type of fertilisation is referred to as internal fertilisation. Factors which increase the probability of fertilisation taking place are the high numbers of spermatozoa released at copulation and the relatively large size of the oocyte. Despite the fact that millions or even billions of spermatozoa are deposited in the female tract, only hundreds of spermatozoa reach the site of fertilisation. Involvement of more than one spermatozoon in fertilisation (polyspermy) is an abnormal occurrence in mammals and invariably leads to early embryonic death. Accordingly, the female tract controls the transport of spermatozoa so that the number reaching the site of fertilisation is sufficient to fertilise oocytes released from the ovary without the likelihood of polyspermy.