Hunnewell v. Cass County was a United States Supreme Court case that dealt with the issue of whether a county could be held liable for damages caused by a defective bridge. The plaintiff, Hunnewell, was injured when his horse and wagon fell through a bridge that had been built by the county. The county argued that it was not liable for the damages because it had not been negligent in constructing the bridge. The Supreme Court held that the county was liable for the damages caused by the defective bridge. The Court reasoned that the county had a duty to maintain the bridge in a safe condition and that it had breached this duty by failing to inspect the bridge and repair any defects. The Court also held that the county was liable for the damages even though it had not been negligent in constructing the bridge. The Court's decision in Hunnewell v. Cass County established that a county can be held liable for damages caused by a defective bridge, even if the county was not negligent in constructing the bridge. This decision has been cited in numerous cases since then, and it has been used to establish the principle that a county can be held liable for damages caused by a defective bridge, regardless of whether the county was negligent in constructing the bridge.

Justice Field delivered the dissenting opinion in Hunnewell v. Cass County, arguing that the majority's decision was incorrect and should be reversed. He argued that a county cannot be held liable for damages resulting from its negligence when it is acting as an agent of the state government. The county had been authorized by statute to construct a bridge over a river, and while doing so negligently caused damage to property owned by Hunnewell downstream of the bridge construction site. Field contended that since this action was taken pursuant to statutory authority granted by the state legislature, any liability arising from such actions must rest with the State itself rather than with individual counties or their officers who are merely carrying out orders on behalf of their principals (the State). Furthermore, he noted that if counties were allowed to be sued for damages due to negligent acts committed while performing duties delegated by higher authorities then they would have no incentive whatsoever to carry out those tasks properly or efficiently; thus undermining public policy goals set forth in statutes granting them such powers in first place.