Recent progress in geophysical and geochemical studies has brought us far in the understanding of the structure, origin, and evolution of the lithosphere. The goal of this book is to summarize geophysical (and, to some extent, geochemical) data collected in the laboratory and in the field on the properties of the lithosphere. It reflects the state of the present understanding of the lithosphere structure and the processes that formed and shaped it. As any other book, it reflects the author’s interpretations that may not necessarily be shared by other researchers. It also reflects the author’s particular interests and, for this reason, the book has a strong focus on the lithospheric mantle, while the crustal structure is discussed in significantly less detail. The motivation for this discrimination is that, owing to historical reasons, the crustal structure is much better known and is much better understood than the structure of the lithospheric mantle. While it is universally understood that the crust is highly heterogeneous, many geophysical models still treat the lithospheric mantle as an almost homogeneous layer.