Section 1983 provides a civil action against a person who, acting under color of state law, deprives someone of a federal constitutional or statutory right. It is the vehicle for most constitutional claims against state and local officials, and two doctrines shape nearly every case.

Qualified immunity shields individual officials from damages unless the conduct violated a right that was clearly established at the time — meaning existing precedent had placed the question beyond debate for a reasonable official. The practical effect is that the analysis often turns on whether a sufficiently similar case had already been decided, and the defense is available at an early stage with an immediate appeal if denied.

Municipal liability works differently. A public entity is not liable simply because its employee violated someone’s rights; there is no respondeat superior under the statute. Liability requires that the violation resulted from an official policy or custom — a formal policy, a decision by a final policymaker, a widespread practice amounting to custom, or deliberate indifference in training or supervision.

That structure explains the shape of these cases: a claim against an officer personally faces immunity, and a claim against the city faces the policy requirement. Claims that survive usually have evidence about the institution — prior complaints, training records, patterns — rather than only about the incident.