The concept of key characteristics deals with the particular moods which different tonalities are believed to provide to music. Discussions regarding their existence and the validity of the phenomena have always been controversial because of a lack of fundamental reasons and explanations for them. Nevertheless, references to key characteristics have appeared in various fields of study and over many centuries: the Greek doctrine of ethos, writings of Guido d’Arezzo, Jean-Philippe Rameau’s Traité de l’harmonie, scribbles in Beethoven’s sketches, and several passages in Hermann von Helmholtz’s On the Sensations of Tones. The attitudes and opinions towards key characteristics have varied in each period of its history. Among the ancient Greeks and Romans, the characteristics of modes were discussed among philosophers, namely, Plato, Aristotle, Lucianus and Cassiodorus. They were believed to affect moral development but were also associated with mysticism. In the Middle Ages and Renaissance, references to key characteristics can be found in the writings of numerous theorists, including Gioseffo Zarlino, Ramos de Pariea and Heinrich Glarean. The studies and discussions of key characteristics in those periods became so well explored as to result in the first appearance of a list of the characteristics of each mode. In Germany and France especially, the discussion of key characteristics reached its peak in the first half of the eighteenth century, when it was studied as a part of Rhetoric. Theorists and composers equally showed their interest in the elements each key could offer to music and how to use keys advantageously in order to enrich the musical experience of the listener. While key characteristics were studied commonly as a vital subject by composers in the eighteenth century and as a fundamental of musical education by many young musicians in the early nineteenth, this tradition had all but disappeared by the middle of the twentieth. The concept of key characteristics is no longer commonly taught in our musical institutions, and this desertion from such a traditionally significant discipline is ever puzzling and particularly interesting to me. In my thesis, I will focus on writings from the last half of the nineteenth century and the first half of the twentieth to determine the various paths taken in the study of key characteristics. I will investigate the writings and discussions of three scholarly groups—music theorists, composers and scientists—from late nineteenth and early twentieth centuries and discuss how the survival of the study of key characteristics was influenced by aspects of the time: popular aspects and aims in the fields of music theory; cultural and social expectations in the validity of phenomena; pronouncements of composers (Arthur Bliss, Alexander Scriabin, Olivier Messiaen, Arnold Schoenberg and Vincent D’Indy) in their musical styles; the rise of a naturalistic view of physical reality as a field and changes it brought to music and societies. I will also include a comparative summary of the status of key characteristics in various periods.