In the historical struggle between Marxism and anarchism, Lenin played an important role. In "Anarchism and Socialism", written in 1901, Lenin denounced anarchism as a petty bourgeois ideology. Lenin defended this view over the next twenty years, as he fought for Bolshevik hegemony in Russia. This article argues that Lenin's struggle against anarchism was significant for several reasons. First, it clarified the fundamental differences between anarchism and Marxism. Second, Lenin contributed to the victory of Marxism over anarchism, initially, in revolutionary Russia, and after that, within the Soviet era communist movement. Third, Lenin's struggle offers original insights. For one thing, Lenin delineated the revolutionary limitations of anarchism. He identified the circumstances in which anarchism can empower or weaken the working class. Lenin also established the organisation, struggle, and leading role of the vanguard working class party as an independent distinction between Marxism and anarchism, a distinction that needs resurrecting today.