The aim of this thesis is to analyze the effect of population ageing on pension system in the Republic of Croatia. Moreover, the emphasis is on the analysis of demographic and macroeconomic assumptions and projections of selected indicators, and the implications that population aging has on the pension system. The issue of population aging is a current topic that the countries of the European Union and the rest of the world are increasingly facing. Numerous countries have tried to solve the problem of the pension system deficit, which has become one of the largest expenditures in state budgets. The Republic of Croatia is trying to keep up with the rest of the countries by introducing reforms and policies, but so far without significant impact on solving the problem. For the Republic of Croatia, as well as most countries of the European Union and the world, it is extremely important to influence this phenomenon in a timely manner. Today, the dynamics are changing leading to the confrontation of pension systems with an uncertain future, which has contributed to the motivation to write this paper. The paper is based on the Aging Report, which helped to analyze the situation in the Republic of Croatia, and accordingly provides an overview of the possible effects of the reform recommendations. Furthermore, since Covid-19 pandemic crisis is currently present in Croatia, an overview of the calculation of the expected projections of the pension system deficit in the short term is given, adjusted to the impact of Covid-19.