The way the pension system works is that a person pays contributions from and on his salary during his working life. The aim of the pension system is to overcome social differences and strengthen social solidarity among the population. Therefore, it is an integral part of the system of any country. In this paper, the pension system of the Republic of Croatia is presented. The pension system consists of three pillars - the first and second pillars represent the mandatory pension insurance, and the third pillar offers citizens the opportunity to save, i.e. it is a voluntary pension insurance. Before describing the pillars of the pension system in more detail, the history of the pension system is presented with emphasis on pension reforms. An employed person has the right to choose a mandatory pension fund to pay into. He or she designates a category within the fund that is associated with certain risks and returns. Certain institutions are responsible for the successful functioning of the pension system. For the research part of the paper, four countries of the European Union were selected - Germany and Ireland as developed countries and Greece and Romania as less developed countries. Gross domestic product per capita is used as a development criterion for classification in this study. Based on these countries and the Republic of Croatia, a comparison of pension funds is made. The comparison shows the investment rules for each country, the distribution of the investment portfolio of the pension systems, the pension fund statistics, the number and assets of the pension funds, the number of members and beneficiaries of the pension funds, and the distribution of assets. The comparison shows that the pension systems of the selected countries and the Republic of Croatia have similar characteristics. Each country has a different investment portfolio of pension funds, but the percentages are similar in some countries.