In 1911 was founded in Bilbao the union Eusko Langileen Alkartasuna (ELA). This book tells us its story during the turbulent and beneficial Republican period from April 1931 until the outbreak of the Civil War in July 1936. It explains the story of a resurrection, after the dark and difficult years of the dictatorship of Primo de Rivera, and an extraordinary expansion that turned ELA into a mass union endowed with a high degree of association and democratic maturity. It is the story of a deep ideological and programmatic renewal, a "union evolution" that contributed to the gradual transformation of ELA into a modern working class organization, politically autonomous and demanding.