In this systematic study of Rosa Luxemburg’s ideas, Isabel Loureiro demonstrates, from a Brazilian perspective, the continued relevance of the most important Marxist theorist of the early twentieth century. Her revolutionary thought, marked by dilemmas and tensions, such as those between laws of history and revolutionary will, rejection of terror and defense of revolutionary violence, shows how Luxemburg’s struggle to build a socialist democracy based on a broad, conscious popular mass remains until today an unresolved problem for the anti-capitalist left.