This survey of opera, organized by century, considerably supplement the coverage of the genre in The New Grove Dictionary of Music and Musicians. The four main section- Baroque, Precalssical and Classical, Nineteenth Century, and Twentieth Century - that follow Stanley Sadie's introductory essay on the nature of opera, are devoted to national developments within the chronological framework; between these are a series of entractes designed to place opera in a broader social and cultural perspective and to convey something of the nature of the world of opera and its changing milieu.