United States v. Wormer was a United States Supreme Court case that dealt with the issue of whether a person could be convicted of a crime if the evidence used to convict them was obtained through an illegal search and seizure. The case involved a man named Wormer who was arrested for the possession of counterfeit money. The arresting officers had searched Wormer's home without a warrant, and the evidence they found was used to convict him. The Supreme Court ruled that the evidence obtained through the illegal search and seizure was inadmissible in court, and that Wormer could not be convicted on the basis of it. The Court held that the Fourth Amendment of the United States Constitution protects citizens from unreasonable searches and seizures, and that any evidence obtained through such searches and seizures is inadmissible in court. This ruling established the precedent that evidence obtained through illegal searches and seizures is inadmissible in court, and that citizens are protected from unreasonable searches and seizures by the Fourth Amendment.

In United States v. Wommer, the Supreme Court was tasked with determining whether a defendant could be convicted of an offense that had been committed before Congress passed a law making it illegal. The majority opinion held that the defendant could not be convicted because there was no existing law at the time of his alleged crime and thus he did not have fair warning that his conduct would constitute criminal behavior. Justice Field dissented from this decision, arguing that Congress has authority to retroactively make certain acts criminal offenses and therefore individuals should still be held accountable for their actions even if they occurred prior to any legislation being enacted. He argued further that allowing people to escape punishment simply because laws were not in place when their crimes were committed would create an incentive for them to commit more unlawful acts without fear of consequence since they know they cannot be punished after-the-fact.