Even though analysis of political institutions – their history, legal and political foundation, and systemic significance – are more and more often treated as a kind of anachronism, this type of research is without any doubt the starting point for any reflection on individual political systems. In our view, institutions contain tradition, reflect the mentality of social groups, customs, ways of conduct, sanctioned by legal or moral norms, respected in each society, as well as reflect the socio-political realities in which they exist. Thus, the process of institutional changes, which is the main axis of the analysis in this chapter, is a key aspect of subsequent civilization evolutions. The twentieth-century history of Poland abounded in several institutions essential for the functioning of the political system, however, in our opinion, two deserve a detailed reflection. The first will be the Senate, which is part of the legislative power, which underwent significant transformations in the 20th century (or even liquidation in a certain period, which was an expression of the prevailing ideology). The second is the institution of the head of state (both in an individual and collegial form), which is part of the executive branch, which, like the upper chamber of the parliament, was subject to a number of changes resulting from the prevailing socio-political realities and the ideological foundations of political systems. The starting point for the analysis below is the hypothesis that, despite several transformations in the Polish state, it was the classic institution that reflected the social and political realities of individual periods in the history of the twentieth-century Poland, as well as the ideological foundations for given political systems crystallised within them. Due to the above observation, in our analysis we will use not only a historical approach to selected institutions, but also a normative and cultural approach. In this perspective, it will be crucial, apart from the normative conditions, to look at the phenomenon of focusing by institutions the most important constitutive symbols and ideas for particular periods in the history of Poland. The analysis will be located between the legal and constitutional approach focusing on the reconstruction of legal norms regulating the field of politics and explaining the relationships between them, and the approach to the science of politics, which emphasizes the description and understanding of this reality.