Robert Michels (; 9 January 1876 in Cologne, Germany – 3 May 1936 in Rome, Italy) was a German sociologist who wrote on the political behavior of intellectual elites and contributed to elite theory. He is best known for his book Political Parties, published in 1911, which contains a description of the "iron law of oligarchy." He was a friend and disciple of Max Weber, Werner Sombart and Achille Loria. Politically, he moved from the Social Democratic Party of Germany to the Italian Socialist Party, adhering to the Italian revolutionary syndicalist wing and later to Italian Fascism, which he saw as a more democratic form of socialism. His ideas provided the basis of moderation theory which delineates the processes through which radical political groups are incorporated into the existing political system.