Castration is any action, surgical, chemical, or otherwise, by which an individual loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemical castration uses pharmaceutical drugs to deactivate the testes. Castration causes sterilization (preventing the castrated person or animal from reproducing); it also greatly reduces the production of hormones, such as testosterone and estrogen. Surgical castration in animals is often called neutering.

The term castration is sometimes also used to refer to the removal of the ovaries in the female, otherwise known as an oophorectomy, or the removal of internal testes, otherwise known as gonadectomy.

The equivalent of castration for female animals is spaying. Estrogen levels drop following oophorectomy, and long-term effects of the reduction of sex hormones are significant throughout the body. Castration of animals is intended to favor a desired development of the animal or of its habits, as an anaphrodisiac or to prevent overpopulation.

The term castration may also be sometimes used to refer to emasculation where both the testicles and the penis are removed together. In some cultures, and in some translations, no distinction is made between the two.