Incest ( IN-sest) is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by affinity (marriage or stepfamily), adoption, or lineage. It is strictly forbidden and considered immoral in most societies, and can lead to an increased risk of genetic disorders in children.

The incest taboo is one of the most widespread of all cultural taboos, both in present and in past societies. Most modern societies have laws regarding incest or social restrictions on closely consanguineous marriages. In societies where it is illegal, consensual adult incest is seen by some as a victimless crime. Some cultures extend the incest taboo to relatives with no consanguinity such as milk-siblings, step-siblings, and adoptive siblings, albeit sometimes with less intensity. Third-degree relatives (such as half-aunt, half-nephew, first cousin) on average have 12.5% common genetic heritage, and sexual relations between them are viewed differently in various cultures, from being discouraged to being socially acceptable. Children of incestuous relationships have been regarded as illegitimate, and are still so regarded in some societies today. In most cases, the parents did not have the option to marry to remove that status, as incestuous marriages were, and are, normally also prohibited.

A common justification for prohibiting incest is avoiding inbreeding: a collection of genetic disorders suffered by the children of parents with a close genetic relationship. Such children are at greater risk of congenital disorders, death and developmental and physical disability, and that risk is proportional to their parents' coefficient of relationship—a measure of how closely the parents are related genetically. However, cultural anthropologists have noted that inbreeding avoidance cannot form the sole basis for the incest taboo because the boundaries of the incest prohibition vary widely between cultures and not necessarily in ways that maximize the avoidance of inbreeding.

In some societies, such as those of Ancient Egypt, brother–sister, father–daughter, mother–son, cousin–cousin, aunt–nephew, uncle–niece and other combinations of relations within a royal family were married as a means of perpetuating the royal lineage. Some societies have different views about what constitutes illegal or immoral incest. For example, in Samoa, marriage between a brother and an older sister was allowed, while marriage between a brother and a younger sister was declared as unethical. However sexual relations with a first-degree relative (meaning a parent, sibling or child) are almost universally forbidden.