A statute passed in 1871 to protect freedpeople from the Klan is now the vehicle for nearly every constitutional claim an American brings against a government official. This article explains how Section 1983 claims are actually built, and why so many of them fail for reasons that have nothing to do with what happened. It walks through the elements, the meaning of acting under color of law, and the constitutional standards that govern force, searches, and conditions of confinement. It explains qualified immunity in operational detail, including the clearly established law requirement that decides most cases. It covers municipal liability, which requires proving a policy or custom rather than a bad employee. It covers Bivens claims against federal officers and the doctrine that has nearly closed them. It closes with damages, fees, and the practical steps that matter in the first week.