Incest is marriage or sexual relations between close relatives. The criminal offense of incest may also be known as prohibited sexual conduct. Laws vary from state to state, but incest laws generally prohibit marriage or sexual relations between (1) a person’s ancestor or descendant by blood or adoption; (2) a person’s current or former stepchild or stepparent; (3) a person’s parent’s brother or sister of the whole or half blood; (4) a person’s brother or sister of the whole or half blood or by adoption; (5) the children of a person’s brother or sister of the whole or half blood, or by adoption; or (6) the son or daughter of a person’s aunt or uncle of the whole or half blood or by adoption.

Lack of consent to sexual relations between such relatives is not an element of the crime, and persons may be guilty of the crime even if both parties consented.

Laws regarding incest or prohibited sexual contact vary from state to state and are generally located in a state’s statutes—often in the penal or criminal code. The crime is a felony offense in many states, with potential punishment of significant jail or prison time.

In California, incest is defined as sexual relations between close relatives, which is illegal under the state's penal code. Specifically, California law prohibits marriage or sexual intercourse between parents and children, ancestors and descendants of any degree, and between siblings whether by the whole or half blood. This also extends to include uncles, aunts, nieces, and nephews. The law is clear that consent is not a defense for incest, meaning that even if both parties agree to the sexual relationship, it is still considered a criminal offense. Incest is classified as a felony in California, and those convicted can face severe penalties including imprisonment.