Pinkerton v. United States is a landmark Supreme Court case from 1946 that established the principle of liability for co-conspirators in criminal conspiracies. The case clarified that all members of a conspiracy can be held liable for the criminal acts committed by one of their fellow conspirators, even if they did not directly participate in those acts. This ruling is crucial in understanding how conspiracy laws can extend liability to individuals who are part of a collective agreement to commit a crime.