Page numbering is the process of applying a sequence of numbers to the pages of a book or other document. The number itself, which may appear in various places on the page, can be referred to as a page number or as a folio. Like other numbering schemes such as chapter numbering, page numbers allow the citation of a particular page of the numbered document and facilitates to the reader to find specific parts of the document and to know the size of the complete text.

The term "folio" has three interconnected but distinct meanings in the world of books and printing: first, it is a term for a common method of arranging sheets of paper into book form, folding the sheet only once, and a term for a book made in this way; second, it is a general term for a sheet, leaf or page in (especially) manuscripts and old books; and third, it is an approximate term for the size of a book, and for a book of this size.