A prenuptial agreement is a contract about a marriage that has not happened yet, negotiated by two people who are not thinking clearly about money, and it is judged years later by a court that will scrutinize it more closely than any commercial contract. This article explains what these agreements can actually do, what they cannot do no matter how carefully drafted, and the four requirements that decide almost every enforceability fight. It follows a hypothetical couple through an agreement signed eleven days before a wedding to show how timing, disclosure, and counsel interact. It explains why the most valuable function of a prenuptial agreement is often not the outcome it dictates but the conversation it forces. It covers postnuptial and cohabitation agreements, which behave differently and are scrutinized differently. It closes with the drafting choices that make an agreement survive and the ones that quietly destroy it.