Persistent identifiers (PIs) are simply maintainable identifiers that allow us to refer to a digital object a?? a file or set of files, such as an e-print (article, paper or report), an image or an installation file for a piece of software. The only interesting persistent identifiers are also persistently actionable; however, unlike a simple hyperlink, persistent identifiers are supposed to continue to provide access to the resource, even when it moves to other servers or even to other organisations. A digital object may be moved, removed or renamed for many reasons. This article looks at the current landscape of persistent identifiers, describes several current services, and examines the theoretical background behind their structure and use. Issues are raised of likely relevance to anybody who is considering deployment of a standard for their own purposes.