A demand letter is the single most cost-effective instrument in pre-litigation practice: a structured written notice that states the facts, connects them to a legal right, makes a specific demand, and sets a deadline, resolving a large share of disputes before a complaint is ever filed. This UpLaw guide dissects the anatomy of a persuasive letter and then treats the letter as what it actually is: a legal instrument with consequences. It works through the evidentiary reach of Federal Rule of Evidence 408 and the settlement privilege, the litigation privilege and Noerr-Pennington immunity that shield a good-faith demand from a defamation counterclaim, and the bright line separating a lawful threat of civil suit from criminal extortion under the Hobbs Act and cases like Flatley v. Mauro. It maps the Fair Debt Collection Practices Act overlay that ensnares lawyers who collect consumer debts, the duty to preserve evidence that a demand triggers on both sides, and the many statutes and contracts that require a written demand before suit. Worked hypotheticals, an annotated model letter, and a deep FAQ round out a research-grade treatment. This article is general legal information, not legal advice.