Given the cross section of people who attend the opera in Europe, you will find that there is no particular dress code. In fact, at the theater you will see a mix of fashions from khakis to suits.
On average, women tend to wear dresses or pant suits while men opt for jackets and ties. The attire for the more expensive seats (namely, the orchestra and the boxes) is a bit more buttoned up with men in suits and women in cocktail dresses. But the farther up you go in the galleries, the more casual the dress will be.
If it’s not a premiere or gala night, an evening gown is not necessary. For these, more casual evenings, there will be people in everything from nice pants and jackets, to ties and skirts. However, Europeans, particularly French people and Italians, tend to dress with a great sense of style, even on informal occasions. With this is mind, you may feel out of place if you dress too casually.Opening Gala Nights
Though what to wear is always up to the individual, opening nights or premieres are special. These formal affairs, which are often attended by dignitaries and celebrities, call for tuxedoes and evening gowns. Teatro alla Scala in Milan, Italy, for instance, has specifically requested that men wear evening dress for premieres. But a jacket and tie is fine for all other performances.