Henri-Pierre Roché's original novel was based on his own experiences as a young man. The original Catherine was still alive when the film was released and even attended the premiere incognito.
Jean Renoir was a particular fan of the film and wrote a letter to François Truffaut expressing his pleasure. Truffaut carried that letter with him for years afterward.
When the production ran out of money, Jeanne Moreau bailed it out with her own personal financial contribution.
Director of photography Raoul Coutard used lightweight photographic equipment that hadn't been used before to create a fluid style to the camerawork, even mounting some of the cameras on bicycles.
The production was so small and had so little money that Jeanne Moreau was often called upon to lend her Rolls Royce for ferrying around props. She even did some of the catering, particularly in the scenes where Jim visits Jules and Catherine in Germany.