Business casual to formal attire is standard for most people attending a ballet performance in the United States. For opening night, more formal attire is appropriate. While people do wear jeans and other casual attire, it is often seen as an example of disrespect to the ballet tradition.
Men should expect to dress in a suit jacket, button-down shirt, slacks and dress shoes for a ballet performance. A tie is a respectful addition, but not necessarily expected. Some men, especially on opening night, wear formal attire, including black tie. This is uncommon for regular-season performances.
Women should expect to dress in a dress, skirt and blouse or in a nice pantsuit. A cocktail dress is respectful but not necessarily expected. Some women, especially on opening night, wear formal-length gowns, though this is not common during the regular season. Many theaters suggest bringing a wrap for open dresses.
Children are the exception to the jeans rule, as long as they are dressed in a tidy ensemble. Generally, parents dress their children as for church or a wedding.
Attending the ballet in Europe is different. People dress up for the occasion. Women wear cocktail dresses and fine jewelry. Men wear full suits or tuxedos. In some venues, men without suit jackets and ties are not admitted. Jeans are not allowed.