A key idea in Daisy Miller is culture clash. As a free-spirited American from New York, Daisy finds European class distinctions difficult to comprehend, as she does the restrictions on women—"ladies"—in Europe. She goes walking and to the Coliseum with Giovanelli, an Italian, because it seems ridiculous to her that she should shun the man just because he is Italian. She also resists the idea that people should be able to dictate to her who she can and cannot see. She doesn't understand that going around—especially alone—with an Italian is the equivalent in her "set" of going around with a black man in the American South at the time. She doesn't understand why this means she must be "cut" by Mrs. Walker.
Another key theme of the novella is innocence destroyed. Daisy represents innocence: she is naïve about European culture and about how easy it is to perish in Europe. She lets romantic ideas of seeing the Coliseum in the moonlight overtake her reason. She dismisses the idea that she could catch malaria doing this, just as she dismisses the dangers of going around with Giovanelli. In the end, her innocence destroys her, both socially and physically.