The lithosphere forms the outer (typically, 50–300 km thick) rigid shell of the Earth. It includes the crust and, in general, some non-convecting part of the upper mantle called the lithospheric mantle (Fig. 1.1). Oceanic lithosphere is recycled into the mantle on a 200 Ma scale, whereas the study of the continental lithosphere is of particular importance since it offers the only possibility of unraveling the tectonic and geologic history of the Earth over the past c. 4 Ga. Knowledge of the structure, composition, and secular evolution of the lithosphere is crucial for the understanding of the geological evolution of the Earth since its accretion, including understanding the processes behind the formation of the early lithosphere, the processes behind plate tectonics, and lithosphere–mantle interaction. Many of these processes are closely linked to processes in the deep Earth and its secular cooling. Human society is strongly dependent on knowledge of the geodynamic processes in the lithosphere which manifest themselves as variations in topography and bathymetry, deposition of minerals many of which occur only in specific lithospheric settings, and high-impact geologic hazards. Understanding processes in the deep Earth is impossible without knowledge of lithosphere structure.