Timbre is an important aspect of music, particularly in the twentieth century. Relatively little serious work on this subject has been published, however, either from a historical standpoint or from an analytical view. The purpose of this present study was to clarify the nature of timbre and to demonstrate the structural role it can have in music. The development of the timbre concept was traced from as early as the fifteenth century to present. From a very simple idea, timbre evolved into a complex phenomenon involving scientific, acoustical analysis and subjective, perceptual evaluation. It was found that currently timbre is not merely a physical manifestation, but also a function of aesthetic judgment and human response to stimuli. A brief historical survey was made of the importance of timbre in music. It was shown that timbre emerged as a significant musical element, forming the characteristic sound structure of a composition and (by extension) the distinctive sonorous style (Klangstil) of a composer. In developing to this point, the idea of timbre metamorphosed into a particular concept of "sound," a term that includes any sound quality that contributes to the character and structure of a composition. A process of analysis was developed to determine musical sound structure. It was shown that the sound-related aspects of the parameters of pitch, dynamics, time, texture, and timbre can act and interact to construct formal shape through cohesion and differentiation. It was established that the music of George Crumb shows a high regard for sound as a structural element. His Night of the Four Moons was analyzed, demonstrating the process of sound analysis. The analysis also showed that various qualities of sound itself are used as compositional material as well as for cohesion and differentiation. This aurally perceived, characteristic structure formed by or related to the action of sound was termed the "audiogenic image."