This Introduction presents the key concerns of “Sonic Things: knowledge formation in flux,” a Special Issue of Sound Studies. Building on recent scholarship in sound studies, history of science, and thing studies, we use the term “sonic things” to describe sounds that stand in for other – elusive, inaudible, or ideal – sounds. Emerging from long processes of multidisciplinary knowledge production, sonic things equally have a quality in and of themselves. The Special Issue’s contributions deal with the histories of sonic things that mediate between physical and metaphysical realms (Leendert van der Miesen, John Durham Peters); the sounds of musical instruments used to reach out to other times or places (Fanny Gribenski, Carmel Raz); and sonic things that moved back and forth between domestic spaces and sites of scientific inquiry (Viktoria Tkaczyk, Tiago de Oliveira Pinto, Flora Dennis). To trace these diverse trajectories, the contributors employ and re-evaluate a plethora of methods, from intellectual history and the history of reception and misconception, to discourse analysis, to the reconstruction of historical instruments and experiments. They speak to readers in a wide range of research fields, offering new perspectives on existing debates and new views on an ample body of historical sources.