Leon Sedov (1906-1938), the first son of Leon Trotsky and his second wife, Natalia Sedova, was born on February 24, 1906, while his father was in a Tsarist prison. He would become Trotsky's closest collaborator in his struggle against Stalin.

Sedov, who was too young to join the front-line battles of the civil war of 1918-21, joined the Komsomol (the youth league of the Communist Party) in 1919. In 1923, at the age of 17, he led the fight inside the Komsomol against the growing bureaucratization of the party and for the policies of the Left Opposition.

He accompanied his father into exile, first to Alma Ata in 1928 and then to the island of Prinkipo in Turkey in 1929. He became editor of the Bulletin of the Russian Opposition and played an indispensable role assisting Trotsky's political work in the difficult conditions of exile. In 1931 he moved to Berlin to continue his university studies and to aid in the political work of the Left Opposition in Western Europe. In 1933, with the coming to power of Hitler, he moved to Paris where he continued to edit the Bulletin and play a leading role in the Left Opposition in Europe.

Sedov's major work, The Red Book on the Moscow Trials, is a concise and damning analysis of the 1936 show trial launched by Stalin against Trotsky and other Bolshevik leaders of the October 1917 Revolution. The Soviet regime sentenced him to death in absentia, along with Trotsky, during the Moscow Trials of 1936-1937.

On February 16, 1938 Sedov died at a Paris clinic, where he went to have an operation to remove his appendix. Mark Zborowski, a GPU agent who had infiltrated the Trotskyist movement, was responsible for his death. The full extent of Zborowski's role in this act was exposed by the International Committee of the Fourth International in its investigation, Security and the Fourth International, the documents of which are available through Mehring Books.