Combining sketch studies, musical analysis, and acoustic analysis, this article examines the impact of spectral models on Grisey’s creative process in the 1970s. First, it focuses on Grisey’s establishment of initial theoretical models in Dérives and Périodes, composed during his residence at the Villa Medici (1972–74), during which period he conducted his own study of acoustics. The article then looks at how, back in Paris, the composer started studying with the acoustician Émile Leipp and then worked with Michèle Castellengo, with whom in 1977 he undertook for the first time the spectral analysis of instrumental sounds. Finally the article examines in detail how some spectrograms, once transcribed, were used in Modulations (1976–77) to construct a complex spectral polyphony. If Grisey draws his inspiration from sound phenomena and the characteristics of auditory perception, he does not for all that neglect structuralism: spectral models serve both to simulate natural phenomena and to formalise his compositional processes.