In the annals of organized crime, the gangsters and mobsters that operated in the United States throughout the early and mid-20th century stand out as some of the most violent and ruthless of recent history. The criminal syndicate known as the Sicilian Mafia, or Cosa Nostra, was especially active during the 1920s, ’30s, ’40s, and ’50s, and enjoyed near legendary status among law enforcement agencies and the public alike. But the era also produced a slew of hardened criminals who took it upon themselves to rob, kill, and intimidate their way across the nation and into perpetual infamy. But who do you think are the most infamous of them all? Click through this gallery of mobsters and gangsters, all of whom thought crime would pay.