Anal sex or anal intercourse is a form of human sexual behavior. While there are many sexual acts involving the anus, anal cavity, sphincter valve and/or rectum, the specific meaning describes the insertion of the penis into the rectum. It is a form of sexual intercourse considered to be particularly risky, for a number of reasons related to the vulnerability of the tissues and the septic nature of the anus.

Such relations have been documented in a wide range of cultures, from earliest times. Where they involved two males, they have also been controversial and sometimes condemned since antiquity. Anal sex is encountered among people of all sexual identities and orientations. While it is reported more frequently among same-sex couples, in absolute numbers, it may be that more heterosexual couples have anal sex than homosexual couples.

In several cultures female receptive anal intercourse in a heterosexual context is widely accepted, especially as there is lower risk of unwanted pregnancy via unprotected anal intercourse (though this is not an absolute guarantee, since semen can leak from the anus, across the perineum, and enter the vagina). Anal sex is even sometimes seen as preserving female virginity, because it leaves the hymen intact. Another reason is that the anus is considered to be "tighter" than the vagina (especially right after a delivery), therefore yielding more tactile pleasure for the penis. The Renaissance poet Pietro Aretino strongly recommended the practice of anal sex in his Sonetti Lussuriosi (Lust Sonnets).

The taboo surrounding anal sex is likely to do with hygiene but also may have its roots in supposed psychoses (deemed responsible for such "deviation"). Some argue that a male heterosexual attraction to the practice has a basis in patriarchal mythologies surrounding a fear of the vagina and suspicion of women's sexual enjoyment and appetites. Additionally, they argue that the appeal of anal sex to many male heterosexuals is a fetish of the taboo, sometimes associated with feces and human waste, as well as of violence and domination, as anal sex practices can result in the bruising and tearing of tissue. Others have argued that the avoidance of the anus is essentially human escapism, a facade whereby man denies his excretory functions, and that, ergo, the practice of the act is merely a form of disillusionment. Moreover, the social taboo surrounding anal sex could potentially be seen as an example of political and religious dogma affecting modern culture, whereby the taboo itself is materialized through initial observance of religious morals, for example those held by some branches of Christianity or Islam.

(A curious pornographic film from about 1980, entitled The Last Days of Sodom and Gomorrah, posits that the 'cities on the plain" were plagued by over-population and threatened with famine. Accordingly, the city fathers made vaginal sex a crime and anal sex a sacrament. Eventually, things went for the worse for the cities and the city fathers.)

From a utilitarian perspective it is also argued by some that the anus is a highly sensitive area with erogenous potential, providing ample opportunity for sexual arousal; that anal sex is a natural permutation of human sexuality, little different from oral sex or other noncoital contact; and that women can derive as much pleasure from the violation of taboos against non-traditional sexual practices as men can. Studies such as that done by Kinsey have suggested that approximately 35-40% of women who have experienced anal sex find it pleasurable, though this figure may vary depending on many factors.

In many cultures, even those where female receptive anal intercourse is considered normal, male receptive anal intercourse, even in a heterosexual context, is seen as taboo, or as less common. In some cultures anal sex is so commonly associated with homosexuality that a male who engages in receptive anal intercourse is considered homosexual, even if he is penetrated by a female partner with a sexual device or other things such as her hand or finger. This is changing with the trend of pegging, when women use devices to penetrate men.