The union of male and female gametes (reproductive cells) during the process of sexual reproduction to form a zygote. It involves the fusion of the gametic nuclei (karyogamy) and cytoplasm (plasmogamy). As each gamete contains only half the correct number of chromosomes, fertilization and zygote formation results in a cell with the full complement of chromosomes, half of which are derived from each of the parents. In animals the process involves fusion of the nuclei of a spermatozoan and an ovum. In most aquatic animals (e.g. fish) this takes place in the surrounding water, into which the gametes are shed. Among most terrestrial animals (e.g. insects, many mammals) fertilization occurs in the body of the female, into which the sperms are introduced. In flowering plants, after pollination, the grain pollen produces a pollen tube, which grows down into the female reproductive organ (carpel) to enable a male gamete nucleus to fuse with the egg nucleus (see double fertilization).

In self-fertilization the male and female gametes are derived from the same individual. Among plants, self-fertilization (also called autogamy) is common in many cultivated species, e.g. wheat and oats. However, self-fertilization is a form of inbreeding and does not allow for the mixing of genetic material; if it occurs over a number of generations it will result in offspring being less vigorous and productive than those resulting from cross-fertilization. In cross-fertilization (also called allogamy in plants) the gametes are derived from different individuals. In plants the pollen comes either from another flower of the same plant or from a different plant (see also incompatibility).