ENAssessment of the issues of the union of Lublin in the historiography of Byelorussia underwent different changes. In the first half of the 19th century it was under the influence of two sovereign historical “schools”: first, an ambience of historians of Vilnius University; second, an ambience of the orthodox and Catholic clergy in Mohylew in the Eastern Byelorussia. Historians of Vilnius University (I. Danilowicz, J. Jaroszewicz, I. Onacewicz) were interested in old history of Russians, but they just analyzed the history of Byelorussian lands as the part of history of Lithuania or Poland. Yet more, they did not assess the union of Lublin. In the end of the 18th century Catholic bishop of Mohylew Jan Bohusz- Siestrzńcewicz published a book “About the West Russia”. For the first time he formulated the thesis, that Byelorussia (Witebsk, Polock, Mohylew) is the west province of Russia. M. Kojalowicz (an originator of ideological movement, which was called as “zapadnoruskij”) developed this theory in the second half of 19th century. Byelorussian historiography and its issues thoroughly built up under the influence of this “school”. The union of Lublin was interpreted as the polonization of political elite, separation of nobility and gentry from lower orders, when over time their origination, language and faith (or rituals) separated them from others. In this concept, the union of Lublin stimulated settlement of Jesuits in the Grand Duchy of Lithuania and the union of Brest. These two events are assessed as Polish expansion to the East with the intention to promulgate Catholicism and polonization of society. That concept appeared in the first handbook of Byelorussian history (Vilnius 1863) which was written by unknown author, in the handbooks of W. Lastowski (1910) and W. Ihnatowski (1919), in subsequent historian works (M. Downar-Zapolski, W. Piczeta, Z. Kopysski, M. Jermalowicz).