In this article, the author outlines a particular mode of composition wherein timbre emerges from micro-level processes of sonic design, on the basis of discrete-time microstructural representations of sound. He examines the notion of timbre as part of this particular mode of composition, suggesting that timbre composition fosters a profound change of compositional paradigm. The discussion includes examples drawn from early electroacoustic works (Pousser, Xenakis, Brün, Stockhausen) as well as more recent work, including the author's own experiences with methods of granular synthesis.