The pension insurance system has only developed as a financial security when it comes to the needs of people for retirement, disability, or the death of the caregiver. From the 19th century to the present, the pension system delegates an indispensable and immensely important role in most modern countries. When it comes to the Republic of Croatia, this kind of system has gone through three reforms and consists of three pillars. It is made up of a combination of intergenerational solidarity and capitalized savings systems. The first and second pillars are mandatory, while the third is voluntary. Consequently, the paper analyzes the pension insurance system on the example of the Republic of Croatia and defines its basic features. The purpose of the work is to highlight the state of the pension system and the future of the pension system in the Republic of Croatia, with special attention being paid to considering its sustainability. To make this possible, the work is an expression of genesis and reform, but also the present state of the pension system in the Republic of Croatia. At the same time, the factors influencing the „instability” pose threats to its sustainability, it is important to emphasize that the data presented and available within the work indicate that the pensions payable based on the insurance from both pension stacks will be considerably lower than the pensions payable since participation only in the first pillar. Given that there are a few reform activities to achieve the adequacy of retirement pensions, the use of which provides for higher pensions with moderate fiscal costs, the question arises as to whether the society and the citizens in a responsible manner share the burden of responsibility for their retirement age. The hypothesis of the work was confirmed in the sense that according to the currently available data presented in the work sustainability of the system is not possible.