Polonization (or Polonisation) (Polish: polonizacja) was the acquisition or imposition of elements of Polish culture, in particular, Polish language, as experienced in some historic periods by non-Polish populations of territories controlled or substantially influenced by Poland. As with other examples of cultural assimilation, it could either be voluntary or forced and is most visible in the case of territories, where the Polish language or culture were dominant, or their adoption could result in gaining of prestige or social status. Such was the case of the nobility of Ruthenia and Lithuania throughout the ages. To certain extent Polonization was also administratively promoted by the authorities, particularly in the period following the World War II.
Polonization can be seen as an example of cultural assimilation. Such view is widely considered applicable to the times of the Polish-Lithuanian Commonwealth (1569–1795) when the Ruthenian and Lithuanian upper classes were drawn towards the more Westernized Polish culture, political and financial benefits of such transition, as well as, sometimes, by the administrative pressure exerted on their own cultural institutions, primarily the Orthodox Church. The conversion to the Roman Catholic faith (and to a lesser extent, Protestant) was often the single most important part of the process as for Ruthenians of that time being Polish culturally and Roman Catholic by religion was almost the same. This aspect of Polonization that led to the diminishing of the Orthodox Church was most resented by Belarusian and Ukrainian masses. In contrast the Lithuanians, who were mostly Catholic, were in danger of losing their cultural identity as a nation, but that did not become evident for the wide masses of Lithuanians until the Lithuanian national renaissance in the middle of the 19th century.
On the other hand, the Polonization policies of the Polish government in the interwar years of the 20th century were again two-folded. Some of them were similar to the mostly forcible assimilationist policies, implemented by other European powers that have aspired to regional dominance (e.g., Germanization, Russification), while others resembled policies carried out by countries aiming at increasing the role of their native language and culture in their own societies (e.g., Magyarization, Rumanization, Ukrainization). For Poles, it was a process of rebuilding the Polish national identity and reclaiming Polish heritage, including the fields of education, religion, infrastructure and administration, that suffered under the prolonged periods of foreign occupation by the neighboring empires of Russia, Prussia, and Austria-Hungary. However, as a third of recreated Poland's population was ethnically non-Polish and many felt their own nationhood aspirations thwarted specifically by Poland, large segments of this population resisted to varying extent policies aimed to assimilate them into Polish culture. Part of the country's leadership emphasized the need for the ethnic and cultural homogeneity of the state in the long term. However, the governmental advancement of Polish language in the administration, the public life and, especially, the education were perceived by some as an attempt at forcible homogenization. In areas inhabited by ethnic Ukrainians for example, actions of the Polish authorities seen as aiming at restricting the influence of the Orthodox and the Ukrainian Greek Catholic Church caused additional resentment, and were considered to be closely tied to religious Polonization, as most ethnic Poles were Catholics.