Originally, and following usual practice, my intention was to write a summary chapter. The topic of the book suggested several options for its structuring: (a) similar to the general structure of the book, by methods and techniques; (b) by major tectonic provinces; or (c) by definitions of the lithosphere and boundary layers. Now, having completed the rest of the book, is it is clear that none of these strategies will be appropriate. On one hand, it will result in significant repetition of what has been already written. On the other hand, lithospheric research remains a very active field with new data and new interpretations coming out on a regular basis. Many issues related to lithosphere structure and evolution are still subject to debate, and new results often provide unexpected evidence for or against highly speculative or well-established concepts. The goal of the book, in any case, is not to promote a certain line of interpretation, but instead to show the complexity of the subject and incompleteness of our present knowledge and understanding of many, sometimes fundamental, issues related to lithospheric processes. For these reasons, any short summary would be a dangerous oversimplification.