U.S. advertising law runs on two engines that look similar and behave nothing alike. The FTC Act, 15 U.S.C. § 45, empowers a federal agency to police deception and unfairness under a low, prophylactic standard that requires no proof of actual injury and no proof of intent. The Lanham Act, 15 U.S.C. § 1125(a), lets an injured competitor sue in federal court for money and injunctions, but only if it can prove falsity, deception, materiality, and its own commercial injury. This guide traces both regimes from statute to case law: the reasonable-consumer standard, express and implied claims, the net-impression rule, the prior-substantiation doctrine and "competent and reliable scientific evidence," unfairness under § 5(n), Lanham standing after Lexmark, literal versus implied falsity and when a survey is indispensable, puffery, establishment claims, comparative advertising, the 2023 Endorsement Guides and influencer disclosure, pricing and negative-option marketing under ROSCA, "Made in USA" and green claims, dark patterns, and the remedial landscape after AMG Capital. It closes with the state-law overlay, the defenses that actually work, and the NAD self-regulatory route.