Published thanks to a research and publication grant from the Canadian Institute of Ukrainian Studies, Hadiats'ka Uniia 1658 roku is a major achievement of international historiography dedicated to Cossack Ukraine. It is a collection of articles by prominent historians from Ukraine, Poland, North America, and Russia devoted to the pivotal event in the history of Cossack Ukraine: the Union of Hadiach of 1658.The Union of Hadiach was a direct alternative to the Pereiaslav Agreement of 1654, for it proposed a different political model for Eastern Europe. Hadiach represented an effort on the part of the Cossack elite to legitimize the Cossack state as the sovereign Grand Duchy of Rus' in the political union with Poland-Lithuania. Despite the failure to implement the Union, its fundamental premise inspired subsequent proponents of Ukrainian statehood and Ukrainian-Polish reconciliation. Apart from the events directly linked to the Union of Hadiach, the collection explores also the political situation prior to 1658 as well as the Union's legacy in Ukrainian and Polish intellectual traditions. This book discusses such topics as the international relations in East-Central Europe on the eve of Hadiach, the genesis of the text of the Union, attitudes of the Cossack, Polish, and Lithuanian elites toward the Union, and the ideological repercussions of the Hadiach agreement in Ukrainian and Polish political cultures.