Leonardo Leo's (1694-1744) career as an opera composer started in 1714, the year of the premiere of his first opera Pisistrato, and continued until 1744, in which his last opera seria Vologeso was written. He worked in a time, in which the musical language of the late baroque gradually changed to the 'early gallant' style. Born in Puglia and educated in Naples, the Composer reacted with the highest sensitivity to this process of change within the musical culture, compared to his contemporaries but in a very individual way. While composers like Leonardo Vinci or Giovanni Battista Pergolesi have completely adopted the new style Leo also integrated older style features into this new style. In the panorama of the 'Neapolitan School' Leo's Drammi seri are fascinating 'hybrids'. To get closer to this special stylistic position of the composer are discussed in this book on the basis of philological studies analysed eight of the 27 complete operas that have survived. Questions concerning the conception and structure of his arias, the comparison of different versions of his works of the compositional process, the phenomenon of self-borrowing and the analysis of the dramaturgy of Leo's stage works. The additionally won philological results are published in a directory of Leos Drammi seri as well as documented in an aria catalogue.