Insemination is the deliberate introduction of sperm into the uterus of a mammal or the oviduct of an oviparous (egg-laying) animal for the objective of impregnating a female for reproduction. Insemination normally takes place during, and as the result of, sexual intercourse between a male and a female, when semen is ejaculated by the male into the female's reproductive tract, but can take place in ways not involving sexual intercourse.

Insemination by sexual intercourse is technically referred to as 'natural insemination' i.e. insemination by natural means. In humans, insemination is most commonly performed by sexual intercourse with a male sexual partner of the female's choice, and sometimes not of her choosing. (See: sexual selection.) It can also be performed by a sperm donor and the term 'natural insemination' or 'NI' is most commonly used when a sperm donor uses sexual intercourse as the method of insemination. In most cultures intravaginal insemination by a male other than the female's normal sex partner is subject to social and sexual inhibitions and taboos, and has legal, moral and interpersonal implications. The incidence of intravaginal insemination by sperm donor is not known, because these arrangements are usually informal and confidential. Whether insemination takes place naturally or by artificial means, the pregnancy and its progress of it will be the same.

Artificial insemination has been and continues to be commonly used in livestock breeding as an efficient way of increasing breeding. With artificial insemination, sperm is used to impregnate the female by direct deposit into the female's reproductive tract. Similar techniques are used in the case of humans, where sperm from a female's male partner or from a sperm donor can be used to impregnate the female. Artificial insemination may be used to attempt to impregnate females to whom insemination by sexual intercourse is not available or is not viable. Examples of these are where the female's male partner is unable to perform sexual intercourse due to physical or physcological difficulties; where the male the partner has died or the partner has some hereditary disability. Artificial insemination is also at times performed if a female cannot, for any of a number of reasons, conceive by sexual intercourse. Most commonly, artificial insemination is used to inseminate females who do not have a male partner, whether single or in a same-sex relationship, and is a method of providing them with their own biological children. In such cases sperm from a man who is a sperm donor, and who will be the biological father of the child, will be deposited in the woman's reproductive tract by artificial means. There are laws in some countries which restrict and regulate who is able to receive artificial insemination.

In various other animal species, sperm is introduced into the female's reproductive tract by various means. For example, sperm can be introduced violently by traumatic insemination, parenteral injection through the body wall, such as in some species of hemiptera. In some species of animals, sperm finds its way through the body wall when the spermatophore is left in contact with the female's skin, such as in the onychophora.