The theory of relativity is one of the fundamental pillars of modern physics and, together with quantum theory, it helps to create the contemporary physical image of the world. In the lecture we will get acquainted with its initial ideas and necessary consequences of relativistic description of particular mechanical processes. The first part of the lecture is devoted to the description from the perspective of inertial systems (special theory of relativity), in the second, we will focus on a general description from the perspective of non-inertial systems, the main consequence of which is a completely new view of gravity (general relativity).