The theory of relativity represents one of the pillars of modern theoretical physics. As the theory of relativity deals with the properties of space and time, it represents the framework for any other theory, especially the quantum mechanics. In the series of articles, we explain the role of the theory of relativity in theoretical physics. The postulates of classical physics are mentioned for didactical purposes, and Einstein’s postulates of the special theory of relativity are introduced. Then, we examine the consequences of the postulates and we translate them into the geometrical language of Minkowski spacetime. It is demonstrated how geometry can help us to understand the relativistic effects (time dilatation) and causal relations between events. Finally, we present the geometrical explanation of the twin paradox. In the following parts of the series, we focus on the general theory of relativity and the relationship between special relativity and quantum mechanics.