The DOI (Digital Object Identifier) is a permanent identifier given to a Web file or other Internet document so that if its Internet address changes, users will be redirected to its new address. The DOI may point to a chapter, an illustration, etc. A DOI must have a corresponding description. An ISBN may have multiple DOI’s. The DOI may contain alpha/numeric characters, however, it must begin with the number 10 followed by a decimal point (10.). The DOI also must contain a forward slash (/) which separates the DOI prefix from the suffix.