Long-term Sustainability of Pension Systems of European Union Countries The article analyses the relationship between population ageing and sustainability of the EU countries' pension systems, and assesses how this relationship is affected by pension system design and parameters. The findings show that there is a link between population ageing and unsustainability of pension systems; however, there are exceptions in both directions. Despite the significant problem of population ageing, pension systems of some countries are found to be sustainable and vice versa. The elements enhancing sustainability are, for example, hypothetical defined contribution accounts or linking the pension benefit or retirement age to demographic changes. The article also analyses the relative significance of population ageing determinants. Life expectancy appears to be the most significant determinant of population ageing.