A prenuptial agreement, often shortened to "prenup," is a legal agreement entered into by an engaged couple prior to being married. The prenuptial agreement provides for an agreed upon distribution of assets if the marriage ends in divorce. A similar agreement called a postnuptial agreement is signed after a couple is already married. Domestic partners may also have a cohabitation agreement.

A prenuptial agreement often has what is called a "sunset" clause. This usually means that the prenuptial agreement is invalid after a certain specified time period, or after the married couple has had a child. Sunset clauses vary from state to state. If the sunset clause is ignored the prenuptial agreement is void. Some couples then construct a postnuptial agreement to change the terms of the prenuptial agreement.

In the US, prenuptial agreements must meet five requirements to be binding. They must be written: oral prenups are not allowed. The signing of the prenuptial agreement must be voluntary on behalf of both parties, and must occur before a notary. The prenuptial agreement can be invalid if one partner fails to disclose something important, and marries under false pretenses. Lastly, the agreement cannot be morally or ethically unconscionable.

Some people and some religions find the prenuptial agreement morally repugnant. It is a statement that allows for the possibility of divorce, and considers what to do in the event of a divorce. Couples may not want to consider how their marriage will end when they intend to be married for life. Churches, like the Roman Catholic Church, don’t ban the prenuptial agreement but certainly do frown upon it. Since a marriage is a lifetime binding commitment and sacrament to the Catholic Church, a prenuptial agreement simply doesn’t make sense and appears to contradict church teachings.

When couples marry and each have significant assets, a prenuptial agreement may be beneficial in keeping those assets divided in the event of the dissolution of the marriage. Couples who have children from previous marriages may also want to use a prenuptial agreement in order to protect assets for their children. In Hollywood, and among the wealthy, the prenuptial agreement is fairly standard fare.

Some are amazed by the provisions included in Hollywood prenuptial agreements. They may specify the minimum number of times in which the couple must engage in intercourse, often called the bedroom clause. They may also have agreements about the wife or husband, maintaining a certain weight or appearance, and agreements about the number of children that will be allowed.

A Hollywood prenuptial agreement might, as well, have an increase in payout to a spouse if the other spouse commits adultery. These terms seem to fly in the face of the “until death do we part,” bit of the marriage ceremony. However, given the short lifespan of some celebrity marriages, a prenuptial agreement seems to make good economic sense.

Depending upon the type of prenuptial agreement, some may be contested during a divorce. Changing circumstances may make for different allocations of spousal or child support if a marriage ends. Most lawyers who specialize in these contracts try to make “airtight” prenups to protect their clients. However, many contentious divorces also include challenges of prenuptial agreements.