United States v. Carll was a United States Supreme Court case that addressed the issue of whether a defendant could be convicted of a crime if the indictment failed to allege the essential elements of the offense. The defendant, Carll, was indicted for the crime of selling liquor without a license. The indictment failed to allege that the defendant had knowledge of the illegality of the sale. The Supreme Court held that the indictment was insufficient because it failed to allege the essential element of knowledge. The Court reasoned that knowledge of the illegality of the sale was an essential element of the offense and that the indictment must allege this element in order to be valid. The Court also held that the defendant could not be convicted of the offense without proof of knowledge. The Court concluded that the indictment was insufficient and the conviction was reversed.

In United States v. Carll, 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 actions to make them criminal offenses. Justice Field dissented from this decision and argued that Congress has the power to pass laws retroactively punishing acts which were previously legal but are now considered criminal due to new legislation. He further stated that such laws should only apply prospectively in order for citizens to have fair warning about what is and isn't allowed under current statutes, but noted exceptions can exist when necessary or beneficial for public safety or welfare.