This movie turned actress Claudia Cardinale into a major star of Italian cinema. During the production, she was 20 years old and secretly pregnant with her first child.
Os Eternos Desconhecidos (1958) was shot in 10 weeks on locations throughout Rome--save for getting through the wall which was done in studio. There is no Madonna Street ("Via Delle Madonne" in the original) in the center of Rome; the robbery takes place in a building at the corner between Via Delle Tre Cannelle e via Della Cordonata, near Piazza Venezia. The apartment and pawnshop were in reality located at 7-8, Via delle Tre Cannelle (41.896613°N 12.486°E), immediately north of Trajan's Market. The building is still standing as of 2019.
Mario Monicelli wanted to call its movie "Le Madame," and only in a second moment changed it into "I Soliti Ignoti" (Usual Unknowns). Le Madame is an Italian slang for Policemen.
At the beginning of the project, Producer Franco Cristaldi didn't want Vittorio Gassman to play his role because he was considered too beautiful and elegant to act like a loser boxer. Gassman acted with very strong make up, included a fake nose, to fill Grimaldi's requests.