The article presents, in a simplifi ed manner, selected theories of population to explain the current trends of population development in Poland and throughout the world. The aim of the article is to present the demographic threats that have emerged in the last eighty years. Prognosticated age structures have also been predicted. Signifi cant diff erences between the structure of the population now and the future have been indicated, particularly regarding the ageing of the population. Against the background of global transformations analyzes of changes in Poland have been conducted. The comparison shows that changes in Poland are highly analogous to transformations around the world, such as declining birth rates and increasing life expectancy. These changes cause ageing of the population, which could result in the collapse of the functioning of the labor market and the pension system as it currently exists.