Sex change is a process by which a person or animal changes sex – that is, by which female sexual characteristics are substituted for male ones or vice versa. Sex change may occur naturally, as in the case of the sequential hermaphroditism observed in some species. Most commonly, however, the term is used for sex reassignment therapy, including sex reassignment surgery, carried out on humans. It is also sometimes used for the medical procedures applied to intersex people. The term may also be applied to the broader process of changing gender role ("living as a woman" instead of living as a man, or vice versa), including but not necessarily limited to medical procedures.

Some species exhibit sequential hermaphroditism. In these species, such as many species of coral reef fishes, sex change is a normal anatomical process.^[1] Clownfish, wrasses, moray eels, gobies^[2] and other fish species are known to change sex, including reproductive functions. A school of clownfish is always built into a hierarchy with a female fish at the top. When she dies, the most dominant male changes sex and takes her place.^[3] In the wrasses (the family Labridae), sex change is from female to male, with the largest female of the harem changing into a male and taking over the harem upon the disappearance of the previous dominant male.

Natural sex change, in both directions, has also been reported in mushroom corals. This is posited to take place in response to environmental or energetic constraints, and to improve the organism's evolutionary fitness; similar phenomena are observed in some dioecious plants.^[4]

Chickens can sometimes undergo natural sex changes. Normally, female chickens have just one functional ovary, on their left side. Although two sex organs are present during the embryonic stages of all birds, once a chicken's female hormones come into effect, it typically develops only the left ovary. The right gonad, which has yet to be defined as an ovary, testes, or both (called an ovotestis), typically remains dormant. Certain medical conditions can cause a chicken's left ovary to regress. In the absence of a functional left ovary, the dormant right sex organ may begin to grow, if the activated right gonad is an ovotestis or testes, it will begin secreting androgens. The hen does not completely change into a rooster, however. This transition is limited to making the bird phenotypically male.^[5] The condition could also be caused by mycotoxins that can develop when animal feed is stored, and these have the same effect as synthetic hormones.^[6] In about 10 percent of cases, if eggs fertilized with male chromosomes are cooled by a few degrees for three days after laying, the relative activity of the sex hormones will favour development of female characteristics. The sex chromosomes work by coding for enzymes that affect the bird’s development in the egg and during its life. This cooling will produce a chicken with a fully functioning and reproductively fertile female body-type; even though the chicken is genetically male.^[7]

Several medical conditions can result in an apparent sex change in humans, where the appearance at birth is somewhat, mostly, or completely of one sex, but changes over the course of a lifetime to being somewhat, mostly, or completely of the other sex. The overwhelming majority of natural sex changes are from a female appearance at birth to a male appearance after puberty, due to either 5-alpha-reductase deficiency (5alpha-RD-2) or 17-beta-hydroxysteroid dehydrogenase deficiency (17beta-HSD-3).^[8][9] A relative handful of male to female changes have been reported, and the etiologies of these are not well understood.^[10][11]

Genetic females (with two X chromosomes) with congenital adrenal hyperplasia lack an enzyme needed by the adrenal gland to make the hormones cortisol and aldosterone. Without these hormones, the body produces more androgens. This causes male sex characteristics to appear early (or inappropriately).^[12][13][14]

Genetic males (with one X and one Y chromosome) with androgen insensitivity syndrome (AIS) are resistant to androgens. As a result, the person has some or all of the physical characteristics of a female, despite having the genetic makeup of a male. The degree of sexual ambiguity varies widely in persons with incomplete AIS. Incomplete AIS can include other disorders such as Reifenstein syndrome which is associated with breast development in men.^[15]

Humans are most commonly said to have "a sex change" when they undergo sex reassignment therapy, that is, a set of medical procedures undergone by transsexual people to alter their sexual characteristics from male to female or from female to male. The term may also refer specifically to sex reassignment surgery, which usually refers to genital surgery only. The term is also sometimes used for the medical procedures intersex people undergo, or, more often, are subjected to as children.

The term "sex change" is sometimes also used for the whole process of changing gender role ("living as a woman" instead of living as a man, or vice versa), not limited to medical procedures. (This process is often much more important to transgender people than the medical procedures themselves, although medically induced changes and surgeries may be needed to make a change of gender role possible, both socially and legally; they can also have a very significant impact on the person's well-being.)

Many people regard the term "sex reassignment surgery" as preferable to "sex change".^[16] Sex in humans is usually determined by four factors:

Not all of these factors can be changed:

Sex reassignment is usually preceded by a period of feminization or masculinization. This is accomplished through hormone replacement therapy, where, for those transitioning to female, estrogens and antiandrogens and sometimes progestogens are prescribed. For those transitioning to male, androgens are prescribed. The most common minimum waiting period before gender reassignment surgery is two years, as specified by the Standards of Care for the Health of Transsexual, Transgender, and Gender Nonconforming People. Hormone replacement therapy is normally started after sufficient counseling, and/or after a period of living 'full-time' (in the target gender) typically for a minimum of six months. This waiting period may vary depending on local regulations and is sometimes nonexistent. Many trans people become medical tourists, as gender reassignment surgery is typically less expensive, less regulated, and sometimes performed by much more experienced surgeons in countries such as Thailand. People who undergo sex reassignment surgery reversal are said to have detransitioned.