The concept of serialism appears conspicuously in the academic literature on twentieth-century music in technical, theoretical, and philosophical contexts. These various contexts, expressed over the course of much of the twentieth century, expose differing connotations of the serial concept. Part I of this chapter explores the serial concept before 1945, reflecting on the multi-dimensional origins of the concept in Arnold Schoenberg’s earliest serial compositions and the significance of Olivier Messiaen’s distinctive serial conceptions prior to the Second World War. Part II explores the serial movement in Europe after 1945, the prominent roles of the journal Die Reihe and the Darmstadt New Music Courses, and the contrasting approaches and attitudes to serialism in the United States after 1945. Tensions between rupture and continuity on both sides of the Atlantic and divergent priorities in discourse about new music demonstrate that theorising serialism entails an understanding of its dynamic disposition, instability, and impermanence.