Under the perspective of the theory of systems and communication theory, this book investigates institucional, political, social and cultural aspects of the opera in Portugal, taking particularly into account the Lisbon Opera House (Teatro Nacional de São Carlos). The opera is seen as a system of social communication in structural connexion with the transformations of the public sphere. From the 19th century to the end of the 20th century, on the basis of a huge amount of empirical sources (archivistic material, periodicals, literary works, legislation, etc.) the author identifies different models of communication whose emergence cannot be separated from the social and political environment. The prevalence of a 'coloquial structure' of communication during the romantic era (until the end of the 19th century), in which the illusionistic presentation of the opera performance as a whole was far from being stabilised, cannot be understood without considering the difficulties of establishing in Portugal a strong bourgeois public sphere, as well as helps to better understand those difficulties. Only at the beginning of the 20th century 'perfect illusion' on the stage and empathy of the audience - the bourgeois model of communication introduced by the enlightened theory and praxis of opera since the 1770's years - has begun to be stabilised in Portugal. Although the enligthened discourse of the educative role of theatre and opera appears in Portugal quite uptodate, in 1771, in a petition of businessmen that Joseph I and his minister Pombal approved, the social and political forces at work did not allow the transference of those ideas into the praxis. With the end of the period of enlightened Despotism and the coronation of Maria I (1777), the influence of the Catholic Church in the Court became so strong as in earlier times, so that the building of the Opera House (Teatro São Carlos) in 1793, a project in which the businessmen of Lisbon were again engaged, had to be justified has a source of income for a charity institution ('Casa Pia'). The enlightened discourse from the 1770's was thus abandoned to give place to the old concept introduced since 1588 by Philip II of Spain, I of Portugal, according to which every theatrical performances were considered forms of entertainment morally suspect that might be only tolerated if part of the revenues revert to charity. The change of the 'coloquial structure' of communication to an effective 'presentational structure', including the ideal of 'perfect illusion' on the stage, the reception of the opera performance as a whole, and the silence, concentration and empathy of the spectator, takes place definitively only after the establishment of the Republic (1910). In this sense, the Republican programme on the cultural field can be seen as a late fully realisation and development of the ideas of the Enlightenment, in that it committed to all art forms a decisive role in the education of the people. The potential of the Republican reforms was, however, interrupted by the Fascist putch of 1926. In the era of 'Estado Novo' the Opera House became above all, since its reopening in 1940, a representative theatre, a kind of 'official reception hall' in which the power and financial elites exhibit themselves and demonstrated their 'heterogeneity' (G. Bataille) before the common people. Such transformations of the sociocommunicative systems are analysed having in view the key motive of the relation music-words in the opera, notably, the lack of a tradition of opera in Portuguese language. Hostility to theater for religious reasons caused the ban of theatre perfomances in Portuguese language from the court since Gil Vicente's dead. Since mid 16th century and until the beginning of 18th century, a kind of court self-censorship did not allow but performances of tragicomedies in Latin language produced by the colleges of Jesuits. Under the reign of João V, from the beginning of the 18th century to 1750, only Italian opera began to be represented at the court. In the meanwhile, retaking a tradition interrupted since Gil Vicente's death, a kind of 'Singspiel' (or musical comedy) in Portuguese language had begun to be represented in 1733 in a public theatre. This musical comedies, with a satirical content and represented by puppets, disappear although tragically from the stage in 1739, when the Inquisition ordered the burning of their author, the playwright António José da Silva, "The Jew" (the author of the music was António Teixeira). As to the Teatro de São Carlos, although being the first State Theatre since its inauguration in 1793, it became an Italian Theatre, in which even Portuguese composers had to write their music on Italian librettos. The whole repertoire was ever sung in Italian language during the 19th century. Despite its 'nationalistic claims' also the Estado Novo did not given any relevant support to the building of a tradition of Portuguese opera. The absence of such a tradition - expressed even in the extreme rarity of translations into Portuguese of repertoire originally created in other languages - is one of the factors that has blocked until today the development of an own opera culture in Portugal.