Improvement Company v. Slack was a United States Supreme Court case that dealt with the issue of whether a patentee could sue for damages for infringement of a patent that had been declared invalid. The patentee, Improvement Company, had obtained a patent for a machine that was used to make improvements to a railroad track. The defendant, Slack, had made and sold a similar machine that infringed on the patent. The Supreme Court held that the patentee could not sue for damages for infringement of a patent that had been declared invalid. The Court reasoned that the patentee had no right to damages for infringement of a patent that had been declared invalid, as the patentee had no right to the patent in the first place. The Court also noted that the patentee had not been able to prove that the defendant had infringed on the patent, as the patent had been declared invalid. The Court also noted that the patentee had not been able to prove that the defendant had acted in bad faith in infringing on the patent. The Court reasoned that the patentee had not been able to prove that the defendant had acted in bad faith, as the patent had been declared invalid. The Court concluded that the patentee could not sue for damages for infringement of a patent that had been declared invalid. The Court reasoned that the patentee had no right to damages for infringement of a patent that had been declared invalid, as the patentee had no right to the patent in the first place. The Court also noted that the patentee had not been able to prove that the defendant had infringed on the patent, as the patent had been declared invalid.

In Improvement Company v. Slack, the Supreme Court was tasked with determining whether a patentee of an invention could bring suit against another party for infringement when that other party had used the patented invention without permission but had not made or sold any products using it. The majority opinion held that such suits were permissible under existing law and granted relief to the patentee in this case. However, Justice Field dissented from this decision on two grounds: first, he argued that Congress did not intend to allow such suits; second, he argued that allowing them would create too much uncertainty and unpredictability in patent law by making it difficult for potential infringers to know what actions might be considered infringing until after they have already been sued. He concluded by arguing that if Congress wanted to grant these rights then they should do so explicitly rather than relying on judicial interpretation of existing laws.