Abstract: This dissertation describes music composition as an act of composing instruments. The building blocks of such instruments are discussed: the fundamentally interdisciplinary approach, the role of gesture, the role of real-time generative software, the mappings between gesture and generative processes, and the interaction between performer and instrument. A real-time performance instrument that was composed to accompany the opera Takemitsu: My Way of Life is described. Key constraints imposed by this project are described, namely: the need for the real-time electronic sound to blend and relate musically to the rest of the music, the need to create a stateless and playable instrument, and the need for an instrument that is robust, adaptable, portable. Design and compositional decisions that address these constraints are proposed and the actual implementation is discussed. As a contrasting example of a composed instrument, a second project is presented: an interactive installation named …in memory of Leah Deni created in memory of Leah Deni. This project serves as an example of the same compositional interest in instrument building and interactivity, but applied to an installation setting where the performer is the audience member. Connections between the conceptual and technological aspects of the installation are drawn. Finally, a set of software modules for real-time creative work named _aLib is presented. The modules in _aLib (a set of abstractions for the Max/MSP environment) were used extensively in the described instruments and will hopefully make a contribution to the real-time computer performance community.