Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A digital object identifier (DOI) is a character string used to uniquely identify an electronic document or other object. Metadata about the object is stored in association with the DOI name and this metadata may include a location, such as a URL, where the object can be found. The DOI for a document is permanent, whereas its location and other metadata may change. Referring to an online document by its DOI provides more stable linking than simply referring to it by its URL, because if its URL changes, the publisher need only update the metadata for the DOI to link to the new URL. However, unlike URLs, the DOI system is not open to all comers; only organizations that can meet the contractual obligations of the DOI system and that are willing to pay to become a member of the system can assign DOIs. The DOI system is implemented through a federation of registration agencies coordinated by the International DOI Foundation, which developed and controls the system.