Before presenting the migrations of populations that took place in the territory of Poland during the years of World War II and during the immediate post-war period, the author analysed the changes of political boundaries that occurred in the territory of Poland between 1939 and 1945. After the statistical and substantive analyses of the forced population migrations, the author presents the detailed demographic balance of Poland between 1939 and 1946, and then again in 1950. The scale of changes that took place is illustrated well by the fact that the population of the Polish state decreased by 11.5 million during these years. This was the result of the changes in boundaries and due to war losses, but it was also the result of the enormous movements of the population. The latter had a selective nature in terms of national structure. This brought about quite essential consequences. Poland changed from a genuinely multi-ethnic country to one of the most homogeneous and mono-ethnic countries of Europe.