Population aging has increased the economic and social demands on economies around the world. It is predicted that by 2050, approximately one third of the European population will be 60 or older (WHO, 2002). As the population ages, the question of greatest concern is how to ensure that their quality of life does not decline compared to that of the previous generation. The decision to retire is an individual decision, but given the significant number of elderly people, it represents a real risk for existing social support systems. In recent years, researchers, public institutions and individuals have become concerned about the sustainability of current pension systems, which represent the most difficult and permanent challenge for developed and developing countries. This is due to an increase in life expectancy together with a sharp decrease in the birth rate, which has led to the search for a new system that will guarantee their sustainability in the future. In order to achieve the sustainability of pension systems, countries often look to each other when reforming their pension systems without recognizing the applicable practices that would contribute to that intention. However, first you need a basic knowledge of the theoretical framework of pension systems. Therefore, the aim of this paper is to provide an overview of the theory of pension systems and to present a comparative analysis of them using the example of Croatia and Slovakia.