The bane of linking is the dreaded invalid URL. The Digital Object Identifier (DOI) helps to maintain the validity of links - at least for those publications that have been assigned the unique identifier by publishers that joined the DOI Foundation. The solution ensures that the links embedded in citations, abstracting-and-indexing records, bibliographies, and any other sources that cite a work or a specific part of it do not become obsolete. More than 250 publishers have purchased DOI prefixes. Even more important, there are now more than 5 million articles registered - but this list does not seem to be complete.