Studies the development and impact of Hanns Eisler's music and Marxist activism in the tensions of 1930s Spain, revealing the interplay of varied influences, ideology and antifascist propaganda. Hanns Eisler in Republican Spain is the first comprehensive study to explore the political, artistic, and intellectual engagements of Hanns Eisler and his circle of Marxist musicians - including the singer Ernst Busch and the musicologist Otto Mayer-Serra - in relation to Spain between 1931 and 1939. The book reconstructs Eisler's collaborations with a broad range of Spanish antifascist organisations, examines the reception of his compositional and theoretical work in Republican Spain, and assesses the deep impact of the Spanish civil war on his vocal and symphonic music. It highlights the influence of key local, national, and international communist structures - notably the Unified Socialist Party of Catalonia (PSUC), the Communist Party of Spain (PCE), and the Comintern (Third International) - on the musical and political projects of Eisler and his circle. Grounded in detailed analysis of an extensive corpus of textual, musical, and press materials - primarily preserved in archives in Spain, Germany, Russia, France, and the United States - this study offers new critical frameworks for understanding the role of Western modernist music in contexts of ideological conflict and war. It provides a fresh perspective on the complex entanglements between antifascist propaganda and musical modernism in the interwar period. Hanns Eisler in Republican Spain makes a vital contribution to scholarship at the intersection of music, exile, propaganda, communism, and antifascism, and more broadly, to the study of how political ideologies shaped music, aesthetics, and musical thought across national boundaries during a pivotal era in twentieth-century European history. On publication this book is available as an Open Access ebook under the Creative Commons license CC BY-NC.