The article deals with the problem of Ukrainian nationalism and its significance in Polish-Ukrainian relations. It considers the role of the Ukrainian Nationalist Organization (OUN) established in 1929, and its militia, the Ukrainian Insurgent Army (UPA), and the genocide committed by these organizations and the Ukrainian population supporting them during the World War II on Poles living in the south-eastern voivodeships of the Second Polish Republic. At the same time, it presents the scientific achievements of the recently deceased Ukrainian political scientist and historian, Associate Professor
Viktor Polishchuk, who, on over 2,579 pages of his works, documented the ideology, organizational structures and genocidal activity of those organizations which, according to the calculations of the Polish side, murdered about 200,000 Poles using the most atrocious methods. The article also lists very critical opinions of the Ukrainian bishop Khomyshyn on the contemporary Ukrainian nationalism, as well as detailed documentation of the crime of genocide carried out by the OUN and UPA on Poles as compiled within the five volumes listed in the bibliography.