Mendelssohn's posthumously published Symphony in A Major has become one of the staples of the nineteenth-century symphonic canon, and is often considered emblematic of Mendelssohn's stylistic strengths and weaknesses - yet the version by which the work has always been known is the first complete version (1833), which was rejected by the composer and rewritten by him the following year. This study documents the Symphony's complicated genesis, examines the reception history of that version, and explores the far-reaching changes incorporated in the 1834 reworking. The book concludes with chapters about the validity and implications of the traditional 'Italian' epithet and the ways in which the music of the two principal versions can shape our understanding of Mendelssohn, his music, and his significance as a cultural figure in the musical culture of the nineteenth century.