This dissertation traces the formation of Kaija Saariaho’s mature compositional practice, culminating in the composition of Lichtbogen (1986), a work that established her as a prominent figure of the European avant-garde and initiated a period of significant artistic productivity. Drawing on various “practice theories,” including Pierre Bourdieu’s Outline of a Theory of Practice (1972), Judith Butler’s performative gender studies (1999 [1990]), and Andrew Pickering’s “mangle of practice” in science studies, I construct a framework for understanding Saariaho’s compositional practice in the context of the material and social affordances of her changing environment. This study is informed by archival materials preserved at the Paul Sacher Foundation and the Institut de Recherche et de Coordination Acoustique/Musique (IRCAM), personal correspondence with Saariaho’s artistic collaborators, and her own published writings and interviews. The dissertation is organized around the first two distinct periods of Saariaho’s compositional development: her student years in Helsinki (1972–81) and her early years in Western Europe (1981–86). Through detailed historical and analytical investigation, I provide the first comprehensive account of Saariaho’s engagement with serial techniques during her studies at the Sibelius Academy (1976–80) and the Hochschule für Musik Freiburg (1981–82). I also explore her little-known involvement in Helsinki’s artistic counterculture, her initial encounters with spectralism, and her experiences at Paris’s institutional centers for avant-garde composition, like IRCAM and the Groupe de Recherches Musicales (GRM). By demonstrating that many of Saariaho’s enduring aesthetic priorities have their provenance in this formative period of her artistic life, this dissertation broadens our understanding of her stylistic influences and complicates the characterization of her music as exclusively “post-spectral.” The final chapter outlines the core features of Saariaho’s mature compositional practice as established by Lichtbogen and shows how they carry through to the new millennium, when her international reputation became established on the basis of her first opera, L'amour de loin (2000). The musical case studies in these chapters allow me to identify several lasting preoccupations in Saariaho’s compositional career, ranging from the role of technology in creative expression to the importance of artistic collaboration and an investment in psychoacoustics. Building on the aesthetic and technical developments that I trace over the course of this formative period in Saariaho’s early career, I suggest that she maintained a lifelong commitment to humanism, reflected in her pursuit of a musical language that expands the boundaries of human experience.