This chapter is an exposition of the meaning of nanometrology and how it ts in with surface metrology. There is a description of concepts rather than the reporting of lists of specications and data. The situation is that the subject is changing so rapidly that factual information quickly becomes out of date; the underpinning concepts have so far had too little time to be formulated convincingly. A primary objective therefore is to consolidate these underlying concepts. However, factual information has not been neglected. It has been distributed appropriately throughout the book rather than being lumped, out of context, in this chapter.