A municipality may be sued for constitutional violations, but not on a theory that it employs the wrongdoer. Liability requires that the violation was caused by an official policy or custom.
Routes to liability. An express policy; a decision by an official with final policymaking authority for the subject matter; a widespread practice so persistent and well-settled that it constitutes custom; or failure to train, supervise or discipline amounting to deliberate indifference.
Final policymaking authority is determined by state and local law, not by job title. A police chief may be a final policymaker for some matters and not others.
Failure to train requires deliberate indifference — usually a pattern of similar violations putting the municipality on notice, or in rare cases a single incident where the need for training was so obvious that the consequence was patently foreseeable.
Causation. The policy must be the moving force behind the violation, which is a stricter standard than but-for causation.
An underlying violation is required. If no constitutional right was violated, municipal liability does not arise regardless of the policy.
No punitive damages against a municipality, and no respondeat superior. Individual defendants remain exposed and may assert qualified immunity, which the municipality itself cannot.