Opera is about people in conflict, sung, played and performed in non-realistic fashion; it does not faithfully imitate real life. I claim that nevertheless, opera can enliven the characters and their state of mind, causing the listener-spectator experience a sensation unequalled by other genres. My aim is to show the musical means by which Berlioz achieved characterization. To do this I select three of his dramatic works, created at different periods of his career, and follow some key figures through the action, analyzing their role. Part One deals with a theoretical approach to characterization and the multiple components of opera. My investigations use the libretto's role only as a basis for the musical events, and exclude the visual element. I present nineteenth-century's aesthetic principles and match them with Berlioz's own credo. I compare some aspects of the novel and poetry with opera. From these readings one can sense Berlioz's urge to express inner feelings, their `psychological essence'. I consider contemporaries' reception of his operas that shows that he was appreciated mainly by a few but first-rank artists. Criticism over the last century is also reviewed, and an assessments made of the composer's own aesthetic position. Part Two provides the Case Studies of characters and deals with the operas Benvenuto Cellini, La Damnation de Faust, and Les Troyens. I also demonstrate the characterization of different atmospheres and of whole operas. In Benvenuto Cellini I concentrate on the role of Teresa, which shows imaginative use of a motive that represents the essence of her character. This method comes close to a 'Leitmotif. In La Damnation de Faust there is a focus on the supernatural, relating to Mephisto. Music is Margarita's natural way of expression and personifies her chaste character. Her music contrasts starkly with Mephisto's. In this unstaged opera music plays a special dramaturgical role. In the opera Les Troyens Aeneas is characterized as a fully rounded and complex person. Music takes an active part in the unfolding of Aeneas's development, as he assumes leadership; in each phase of his development, in intimate situations or in authoritative ones, Berlioz found the adequate musical idiom to deepen our comprehension of his motivations. In conclusion: Characters achieve a `psychological essence' because they appear as human beings with weaknesses and virtues. Berlioz applied no single method, but a deep understanding both of human nature and of the language of music. It is possible to follow the composer's intentions by listening attentively to the symbolic language in which they are offered.