This thesis studies the Heroic-Romantic opera Fierrabras, Schubert's most ambitious theatrical work, which was not created during the composer's lifetime and had to wait for a long time a partial rehabilitation. The first part studies the context around the work's genesis in 1823. Schubert's project is examined in face of the reality of the musical life in Vienna, of the different problems (search for new librettos, spoken dialogue) created to composers by the attempt to elaborate an original german opera. The second part is focused on the text of the libretto. The course and the transformations of the two stories which inspired the subject, are recounted until their reception by the Germanic nineteenth century. The survey of the literary sources of the libretto shows what part in the handling of the subject comes from the conventions of the time and what are the librettist's and composer's personal contributions. Themes and ideas close to Schubert and his friends are important in the text. It indicates the peculiarity of the libretto of Fierrabras, which the external setting is the medieval chivalry. The musical analysis of the opera shows some specific stylistic references which don't prevent Schubert's personal style from appearing plainly in many aspects of the language, deserving an original dramatic strategy. Schubert masters and combines all the techniques of composition used in the theatrical music of his time. But he also introduces innovation for exemple in the construction of complex scenes, with great dramatic effect, integrating melodramas. At the time where german opera experiments, he realises with Fierrabras an original work, also looking ahead.