Robert Michels (1876-1936) is best known for his 1911 book Political Parties, which is still a standard reference in political science debates. Michels' work sought to prove an "iron law of oligarchy" that governs the organisational evolution of democratic political parties. The work was closely informed by Michels' engagement with the German Social Democratic Party in the early 1900s, his involvement in radical politics in France and Italy in this period, and by his interest in a range of intellectual and social movements - including feminism, nationalism, racial theory, and the emerging disciplines of sociology and political science. Using archival and printed sources hitherto overlooked in work on Michels, this new study contests previous arguments which have sought to explain Michels as a disillusioned adherent of ideas of direct democracy or as an extremist moving from revolutionary syndicalism to fascism. The biographical and intellectual influences on Michels are shown to be more complex, and more transnational, than such schematic explanations have allowed. Andrew Bonnell sheds new light on Michels' relationship with the German Social Democratic Party and on his understanding of his own role as an intellectual in a workers' party. Bonnell also analyses Michels' problematical relationship with revolutionary syndicalism in France and Italy. Michels was connected to a possibly uniquely diverse network of intellectual and political contacts in pre-1914 Europe. This transnational intellectual history illuminates the intellectual worlds in which Michels moved and presents a new interpretation of his shift from the radical left of the spectrum to Italian fascism, an intellectual itinerary which has intrigued many historians.