Leon Trotsky and Victor Serge represent the great and tragic oppositional figures to Stalin's dictatorial grip on the Soviet Union in the late 1920s and 1930s. Written during this period, the letters exchanged between these two friends, published here in translation for the first time together with other material from both the Trotsky Archive at Harvard and the Serge Archive in Mexico, present a unique first-hand account of the alternatives and arguments of the Trotskyist opposition in exile. The correspondence chronicles Trotsky's attempts to found a new Fourth International and casts new light on the trajectory of the Russian revolution from Lenin to Stalin and the long term effects of Stalinism for the revolutionary movements in the West. A remarkable insight into the lives of two prominent thinkers of the twentieth century, these letters also help us to understand an important relationship during a critical period in European politics. Each section is prefaced by a clear introduction that contextualises and clarifyies the documents.