The most consequential question in American criminal procedure is asked by an officer at a car window, and most people answer it wrong. This article explains what the Fourth Amendment actually requires, starting with the question of when a search happens at all, and moving through the enormous set of exceptions that swallow the warrant requirement in practice. It covers consent and why it is the single most common basis for a search, the automobile exception and its limits, searches incident to arrest and the digital carve-out for phones, plain view, exigency, and the protective frisk. It explains what a traffic stop may and may not become, why an officer's actual motive is largely irrelevant while the duration of the stop is decisive, and how the exclusionary rule has narrowed to the point where a violation frequently produces no remedy at all. It closes with what a person can say at a stop and what a lawyer looks for afterward.