In addition to its vital importance for the social and aesthetic development of art music after the Enlightenment, the symphony is also pivotal for the evolution of tonality. As the most prestigious instrumental genre of the nineteenth century, it embodies on the largest scale changes in the way tonal relations underpin instrumental forms. To trace the development of symphonism from the late eighteenth century to the fragmentation of common practice at the start of the twentieth century is in a sense to observe in microcosm the history of tonality in its common-practice phases. Understanding this history requires engagement with a variety of theoretical, technological and historical issues. To the extent that the exploration of chromatic tonal relationships undertaken by composers in the first half of the nineteenth century shadowed an emerging consciousness of tonality as a musical system, it reflects a striking shift of theoretical attitude. Where eighteenth-century theory mingled notions of mode, key and harmonic schema with concepts of topic and melodic rhetoric, nineteenth-century theorists, from Alexandre Choron’s seminal coinage in 1810 onwards, were increasingly concerned with the system of tonality itself. In parallel, symphonists also responded to major advances in instrumental technology and temperament, which are intimately related to the expansion of tonal means gathering momentum by the mid-nineteenth century. In particular, the rise to dominance of equal temperament and the dissemination of instrumental modifications accommodating this change underlies the increasing confidence with which composers constructed forms around remote tonal relationships.