The Digital Object Identifier (DOI®) System is a managed system for persistent identification of content on digital networks. It can be used to identify physical, digital, or abstract entities. The identifiers (DOI names) resolve to data specified by the registrant, and use an extensible metadata model to associate descriptive and other elements of data with the DOI name. The DOI system is implemented through a federation of registration agencies, under policies and common infrastructure provided by the International DOI Foundation which developed and controls the system. The DOI system has been developed and implemented in a range of publishing applications since 2000; by early 2009 over 40 million DOIs had been assigned. The DOI system provides identifiers which are persistent, unique, resolvable, and interoperable and so useful for management of content on digital networks in automated and controlled ways.