This guide covers marital and cohabitation agreements from the first conversation through litigation over enforceability. Part 1 distinguishes the three instruments and explains why they are judged by different standards. Parts 2 and 3 set out the statutory framework and the four enforceability requirements that decide nearly every challenge: voluntariness, disclosure, counsel, and substantive fairness. Part 4 covers what these agreements can do and Part 5 what they cannot, including the federal regimes that private contracts cannot override. Part 6 covers property characterization, commingling, and active appreciation, which destroy more agreements in practice than any doctrine. Part 7 covers spousal support and the second-look problem. Parts 8 and 9 cover postnuptial and cohabitation agreements. Parts 10 through 12 cover drafting, execution, and the anatomy of an enforceability challenge.