Trotskyism is the theory of Marxism as advocated by Leon Trotsky. Trotsky identified as an orthodox Marxist and Bolshevik-Leninist, and supported founding a vanguard party of the working-class, proletarian internationalism, and a dictatorship of the proletariat based on working-class self-emancipation and mass democracy. Trotskyists are critical of Stalinism, as they oppose the idea of Socialism in One Country. Trotskyists also criticise the bureaucracy that developed in the USSR under Stalin.

Vladimir Lenin and Trotsky were close both ideologically and personally during the Russian Revolution and its aftermath, and some call Trotsky its "co-leader". However, Lenin criticized Trotsky's ideas and intra-Party political habits. Trotsky was the paramount leader of the Soviet Red Army in the direct aftermath of the Revolutionary period.

Trotsky originally opposed some aspects of Leninism. Later, he concluded that unity between the Mensheviks and Bolsheviks was impossible, and joined the Bolsheviks. Trotsky played a leading role with Lenin in the revolution. Assessing Trotsky, Lenin wrote, "Trotsky long ago said that unification is impossible. Trotsky understood this and from that time on there has been no better Bolshevik."