A Structure holds things together, carries loads, and provides integrity. Structural engineering is the application of statics and solid mechanics to devise structures with sufficient strength, stiffness, and useful life to fulfill a mission without failure with a minimum amount of weight. Aerospace engineers pay particular attention to designing light structures due to the strong dependence of weight on operational costs. The aim of this chapter is to give an overview of aircraft structures. The chapter starts with some generalities in Section 4.1. Then material properties are analyzed in Section 4.2 focusing on aircraft materials. The Loads that appear in an aircraft structure will be described in Section 4.3. Finally, Section 4.4 will be devoted to describe the fundamental structural components of an aircraft. Thorough references on the matter are, for instance, Megson  and Cantor et al. .