<p>The entire structure of Lie groups can be reconstructed in terms of the so-called simple roots: this basic concept of the group theory is the topic of a detailed discussion in this video. It outlines the reason to introduce the simple roots and their geometrical properties, encoded in mathematical objects such as the Dynkin diagrams or the Weyl group. This video is part of a complete course consisting of 15 videos where there will be covered in details (together with many instructive examples) the basic properties of Group Theory of finite and continuous groups (such as the permutation group and the SU(2) group), the representation theory, the structure theory of roots and weights and the classification of compact Lie group. Symmetry is one of the most pervasive and powerful concepts in mathematics and its language is given by Group Theory, that is a quite universal discipline and finds vast applications in many other subjects, such as many branches of physics. Examples and exercises presented in this lesson will range from number theory to simple physical systems. The ideal viewers are undergraduate and graduate university students of scientific disciplines, postdoctoral fellows and, in general, scientific scholars eager to learn or deepen their knowledge on Group Theory.</p>