The pension system is a part of the social security system, whose primary role is to protect against the risk of old age, the risk of permanent economic incapacity and the risk of poverty for the insured person or his/hers family after his/hers death. In recent decades, the Croatian pension insurance system has been faced with many difficulties in performing the aforementioned functions, and the reasons for this lie in numerous demographic, economic and fiscal problems. Although several reforms of the pension insurance system have been carried out, there are certain factors that prevent its smooth functioning. The aim of this thesis is to present the structure of the pension insurance system of the Republic of Croatia using the inductive and deductive method, the method of analysis and synthesis, and the method of classification and comparison, and to analyze whether it is sustainable in the long term. Although, according to the projections, further unfavorable demographic and economic trends are expected, the costs of the pension system should decrease in the future under the influence of various factors, so it is concluded that, from the fiscal point of view, the pension insurance system of the Republic of Croatia is sustainable in the long term.