The set of vertical and horizontal boundaries defines the building envelope, which plays a crucial role in controlling and regulating energy consumption, acting as a boundary between external and internal spaces. This chapter, after breaking down the building into its various technical elements according to the UNI EN 8290 standard, provides an overview of the requirements and performance of the building envelope, as well as a brief summary highlighting its historical evolution. Starting from the first forms of primitive dwellings to the current ones, the evolution of the envelope allows us to understand the advances and developments in the construction sector in terms of techniques and materials.