Ducat v. Chicago was a United States Supreme Court case that dealt with the issue of whether a city could be held liable for damages caused by a defective bridge. The plaintiff, Ducat, was injured when a bridge owned by the city of Chicago collapsed while he was crossing it. Ducat sued the city for damages, arguing that the city was negligent in maintaining the bridge. The Supreme Court held that the city of Chicago could be held liable for damages caused by the defective bridge. The Court reasoned that the city had a duty to maintain the bridge in a safe condition, and that it had breached that duty by failing to do so. The Court also held that the city was liable for Ducat's injuries, even though the city had not been aware of the defect in the bridge. The Court's decision in Ducat v. Chicago established that cities can be held liable for damages caused by defective public works. This decision has been cited in numerous subsequent cases, and has been used to support the idea that cities have a duty to maintain public works in a safe condition.

In Ducat v. Chicago, the Supreme Court was divided in its opinion on whether a city ordinance that imposed a tax on all persons who kept horses within the city limits was constitutional. The majority of justices held that it did not violate any provision of the Constitution and thus could be enforced by local authorities. Justice Field dissented from this decision, arguing that such an ordinance violated both due process and equal protection clauses of the Fourteenth Amendment as well as other provisions of federal law which prohibited states from taxing citizens without their consent or representation. He argued further that since cities are creatures of state governments, they should not have more power than those governments when it comes to taxation matters; otherwise there would be no limit to what taxes cities can impose upon citizens without their approval or consent.