This lecture introduces the concept of stability in structures, covering topics such as the path of structures, forces, equilibrium, resistance, deformations, and stiffness. It explores the equilibrium in different structural elements like cables, arches, membranes, and shells, as well as the behavior of compressed elements. The instructor discusses the stability of bars, tension, and compression structures, emphasizing the importance of pressure lines in determining stability. The lecture concludes with insights on the position of pressure lines in compressed elements.