Internal fertilization is a form of animal fertilization of an ovum by spermatozoon within the body of an inseminated animal, whether female or hermaphroditic. This is distinct from external fertilization, where the union of the ova and spermatoza occur outside of the organism.

In mammals, internal fertilization is done through copulation, which involves the insertion of the penis into the vagina. Some other higher vertebrate animals (most reptiles, most birds, and some fish) reproduce internally, but their fertilization is cloacal.

The union of spermatazoon of the parent organism. At some point, the growing egg or offspring must be expelled. There are three general ways of doing this:

• Oviparous organisms, including most insects and reptiles, monotremes, and all birds lay eggs that continue to develop after being laid, and hatch later.

• Viviparous organisms, including almost all mammals (such as whales, kangaroos and humans) bear their young live. The developing young spend proportionately more time within the female's reproductive tract. The young are later released to survive on their own, with varying amounts of help from the parent (s) on the species.

• Oviviparous organisms, like the garter snake, and the Madagascar hissing cockroach, have eggs (with shells) that hatch as they are laid, making it look like "live birth".