Collective, or group, improvisation exists in almost every music culture, but its manifestations differ widely. This book proposes terminology, concepts, and methods for examining divergent practices in a unified way, developing and testing broadly-applicable analytical frameworks of improvisatory process and collectivity through musical analysis. At the micro-level, the book offers insight into the note-by-note decisions of improvising performers. Its two central case studies present contrasting Balinese gamelan practices, of the interlocking four-person melodic reyong norot and paired kendang arja drumming, as examples of the book's analytical frameworks in action. Detailed, ethnographically-informed musical analyses uncover models and knowledge bases guiding improvisation in these practices and elucidate idiomatic ways of diverging from them in the course of performance. These case studies are punctuated and complemented throughout by analytical forays into diverse non-Balinese practices, which help refine typologies of collective improvisation with comparative and cross-cultural relevance. At the macro-level, the book illuminates the larger musical, discursive, structural, and cultural factors that shape collectively improvised performances in Bali and beyond, drawing from theories of communication and interaction, creativity and cognition, to unpack how performers play and imagine as a collective. It places collectivity at the center of improvisation discourse and argues that music analysis, framed in specific ethnographic insights but with an eye to cross-genre applicability and comparative analysis, is a powerful tool for exploring complex musical and interactional relationships. And by placing diverse practices on a level playing field for analysis, the book enables deeper insight into collectivity and improvisatory processes across cultures.