This project is an ethnography of DIY music technologists in Berlin and New York City—a constellation of “makers” (inventors, hackers, sound artists, circuit benders, musicians) who build their own experimental musical instruments, gear, and other sound-focused technologies. Exploring the production, circulation, and cultural significance of handmade musical instruments and gear, the book interprets “DIY” as a category of invention and an informal economy between labor and leisure. I trace the creative and commercial tensions of DIY from the emergence of hobbyism as a form of “productive leisure” to the prevailing discourse of punk rock and the adoption of DIY as the rallying cry of the recent Maker Movement. I consider how these music technologists engage artificial intelligence, produce tangible and virtual instruments, and are spearheading a renewed fascination with modular synthesizers. The book investigates the historical dimensions of electronics tinkering behind the Iron Curtain (and the circulation/blockage of materials that occurred), in tandem with a consideration of the contemporary relations in Berlin between makers and technology start-ups, including discourses on crafts-based businesses and the role of makers in gentrification. I argue that the cultivation of the self as a productive cultural citizen—a process of “becoming” that I liken to a state of “permanent prototyping”—is central to my interlocutors’ activities, through which sound, self, and instrument are continually remade.