Inequitable conduct is the litigation defense that can render an entire patent — and sometimes a whole patent family — permanently unenforceable because the applicant broke the duty of candor owed to the U.S. Patent and Trademark Office. This article maps the duty under 37 C.F.R. § 1.56, the 2011 en banc reset in Therasense v. Becton, Dickinson, and the two independent pillars that now control every case: but-for materiality and specific intent to deceive. It works through the narrow egregious-misconduct exception, the use of litigation misconduct to prove prosecution-era intent under Regeneron v. Merus, and the reach of infectious unenforceability across related patents. On the practical side, it treats the Information Disclosure Statement as the day-to-day heart of compliance, supplemental examination under 35 U.S.C. § 257 as the primary post-grant cure, and Rule 9(b) pleading under Exergen as the gate that screens thin allegations. Plain-language explanations and invented hypotheticals keep the doctrine legible for inventors, executives, judges, and counsel alike.