Patent marking under 35 U.S.C. 287 is the cheapest compliance step in patent law and the most expensive one to skip, because an unmarked patentee can be barred from recovering a single dollar of damages for years of infringement, even against a deliberate copyist. This guide separates constructive notice (earned by consistent marking) from actual notice (a specific charge of infringement against a specific product), and explains why marking limits the timing of damages rather than the validity of the patent. It works through the America Invents Act's virtual-marking option, the patentee's non-delegable duty to police licensee marking under Arctic Cat v. Bombardier, and the special rules for method claims, process patents under Section 287(b), and the medical-practitioner carve-out in Section 287(c). It covers the post-AIA collapse of false-marking liability into a competitive-injury claim under Section 292, the good-faith defense of Pequignot, and the residual state-law exposure the AIA left untouched. With laches gone after SCA Hygiene, marking is now the principal limit on how far back a damages claim can reach, and the article closes with worked hypotheticals, a Section 292 checklist, and an auditable compliance program built to survive litigation.