The Mendelssohns were active at a time of contestation and change within music aesthetics and broader aesthetic theory. As well as outlining how they positioned themselves in relation to some of the key issues and debates of their time, the chapter examines their continuing investment in Enlightenment and classical aesthetic ideals and how this interacted with their engagement with Romanticism. It also explores the extent to which moral and aesthetic criteria are entwined in their judgements of contemporary music, fuelling their hostility towards French grand opera, the programmatic orchestral works of Berlioz, and French virtuoso pianism. Their own compositions frequently function as music-aesthetic interventions, aiming to counterbalance trends in musical life that they viewed negatively. Crucial is a discussion of the conceptions of truth and emotion at the heart of Felix’s aesthetics, explored through a comparison of his views with those of Georg Wilhelm Friedrich Hegel and Friedrich Schleiermacher.