The most powerful defense in civil litigation requires no evidence, no witness, and no dispute about what happened. If the clock ran, the case is over. This article explains how limitations periods actually work, which is far less mechanical than the phrase "two-year statute" suggests. It separates the four questions that decide every timeliness fight: which period applies, when the claim accrued, whether anything stopped or extended the clock, and whether a separate statute of repose has cut off the claim regardless of the answers to the first three. It covers the discovery rule and its limits, the fraudulent-concealment and equitable-estoppel doctrines that punish defendants who hide the injury, equitable tolling and its narrow diligence requirement, the continuing-violation doctrine that is invoked constantly and succeeds rarely, and the class-action tolling rule of American Pipe and its limits after China Agritech. The final sections cover repose statutes and why they behave so differently, the presuit notice requirements that quietly shorten claims against public entities and health care providers, contractual limitations clauses, borrowing and savings statutes, and the diary practices that prevent the single most common form of legal malpractice.