A prenuptial agreement is a contract, signed before a wedding and effective on marriage, that lets a couple replace their state's default property and support rules with rules of their own. This article maps what a prenup can and cannot do, then works through the enforceability factors courts apply almost everywhere: a signed writing, voluntary execution, fair financial disclosure, freedom from unconscionability, access to independent counsel, and capacity free of overreaching. It explains how the Uniform Premarital Agreement Act (1983) and the Uniform Premarital and Marital Agreements Act (2012) structure the field, and why states still diverge sharply on the timing of the fairness inquiry, on spousal-support waivers, and on mandatory counsel and waiting periods, with Pennsylvania's Simeone v. Simeone and California's post-Bonds statute marking the poles. It gives extended treatment to postnuptial agreements and to the special problems of founders and creators whose most valuable assets are equity, patents, copyrights, and royalty streams. A detailed hypothetical shows the doctrine working end to end, and a practitioner checklist and FAQ close the piece. Prenuptial-agreement law is intensely state-specific; this is a research overview, not a substitute for counsel licensed where the agreement will be governed.