The book presents the gradual transformation of opera to modern, diversified forms, especially the subgenres which go beyond the accepted, conservative canon through its marriage with popular music styles. Despite their connections with opera heritage, they seem to be ignored and considered as not fully-fledged operas due to their popular appeal and close associations with mass culture. The process of transformation presented in the book covers the period from the activity of the Florentine Camerata and the first performance of Daphne by Jacopo Peri in 1597, to the release of the concept album of the rock-opera Tommy by the British rock band The Who in 1969. Although the research deals with the changes of opera to the creation of the rock-opera Tommy, it presents also other eminent works of this subgenre to illustrate fully its diversity, richness and artistic value. The presented analysis of the transformations of opera from elite art to popular entertainment for people coming from different social background with different music taste, contains three main themes: the form of opera, its soloists and the behavior of audiences.