The aim of this paper is an attempt of explanation basic factors of depopulation in case of cities in Poland in the 20^st and in the beginning of 21^st century. Author takes into consideration only large urban centers with population more than 50 thousands inhabitants. Population changes are described according to historical periods: 1900-1946 (including World War II), postwar period (1946-1989) and period socio-economic and political changes in Poland (1990-2010). A key question is basic thesis that different determinants of depopulation have two sources: internal as well as external ones. Some of them arise only in defined periods and social-economic conditions. The most important have been underlined in this paper. It has been highlighted that is visible different between simply depopulation of cities as consequence of military operations, for example and contemporary urban shrinkage as an effect of compound social-economic background. Paper gives some reflections on different possibilities for defense of cities not only against consequences of the second demographic transition but also functional changes, especially in case of larger post-industrial centers. Important question that have been underlined in paper is problem of urban shrinkage as contemporary dimension of depopulation. Presented information clearly shows that in demographic terms process of contemporary shrinkage is comparable with urban demographic decline of the WW2 period.