Del Shannon re-wrote and re-recorded his classic hit "Runaway" for the title sequence of the series.
John Santucci, who portrays Ray Luca's sidekick Paulie Taglia, was a real-life professional thief, who was busted on numerous occasions by series co-creator Chuck Adamson and Dennis Farina when they were Chicago cops. Many of the robberies, hijackings, and burglaries are based upon Santucci's crimes, including the museum jewelry heist profiled in the pilot, according to the DVD liner notes.
Nate Grossman was based upon a real detective of the same name, who worked with Chuck Adamson and Dennis Farina. In real life, Grossman was killed in the line of duty.
By the time season two began, Paulie Taglia had become so popular as comic relief that John Santucci finally got his name on the opening credits.
Ray Luca was based upon Chicago mobster Tony "The Ant" Spilotro, who also inspired Joe Pesci's character of Nicky Santoro in Casino (1995). By the same token, Max Goldman, and Robert De Niro's character of Sam "Ace" Rothstien were both based upon Spilotro's partner, Frank "Lefty" Rosenthal.