In this article, the author investigates the roles of synthesis in the development of musical thought from the late nineteen-forties to the early nineteen-seventies, in a way that avoids a fetishisation of synthesis/synthesiser technology by focussing on synthesis as a concept. He concentrates on developments originating in what is commonly regarded as the electroacoustic music of the European avant-garde, in particular, two approaches that he characterises as French (Pierre Schaeffer - musique concrète) and German (Karlheinz Stockhausen - elektronische Musik) synthesis. He addresses several questions: what was understood by the concept "synthesis"? Why did it develop in different ways? And what are its ramifications for contemporary musical languages?