The thesis is focused on the analysis of the sustainability and adequacy of the Belgium pensions system. The pension system is a system that functions in such a way that during their working life, employees and their employers pay contributions from and to salaries. In fact, it is necessary for the pension system to function properly because the system provides social security to the elderly when the elderly are no longer able to earn a living on their own. The purpose of this thesis is the historical overview of the development of the pension system in Belgium, includes changes in the introduction of the necessary legal acts and pension reforms, which confront the system with demographic changes and the costs of financing the system, and also presents the current organization of the system. The Belgian pension system is a three-tier pension system, consisting of public, supplementary (extra-legal) and voluntary(individual) pension savings. Also, the three pillars of pension insurance include state pensions, occupational pensions and private property. The first pillar of pension insurance covers a statutory or public or state pension. The second pillar of pension insurance includes occupational pension, ie. supplementary (extra-legal) pension, in which employers voluntarily pay for the pensions of their employees. Third pillar includes personal pension savings. Also, to analyze it through various indicators on which the pension system depends, such as the possibility of preventing poverty, the level to which pensions can replace income, achieved pension rights and its duration, and based on them to evaluate financial sustainability and adequacy. By determing the concept of sustainability and adequacy, it can be concluded that demographic indicators and financial condition have a great influence on the long term sustainability and adequacy of the system. In order for the pension system to be sustainable in the long run, it must find a way to influence demographic change or the aging process without weakening public finances.