This is an investigation of the modern political history of the Rhondda Valleys, from the late 19th century through to the aftermath of World War II. Rhondda returned Wales's first Labour MP, nurtured doctrines of workers' control of industry, and became a Labour Party stronghold before 1914. In the inter-war decades Rhondda's political culture sustained not only a resilient Labour Party, but also provided for a level of Communist Party support umparalleled in British terms, leading to fierce political conflict between the two left-wing parties. This study aims to provide an explanation for the Labour Party's rise to power and subsequent domination in the Rhondda.