A repository is a central place in which an aggregation of data is kept, organized and maintained.  Open access repositories provide free and unrestricted access to scientific and research material (full text and metadata). Open access repositories have their roots in initiatives related to the free disposal of files and content such as Open Archives Initiative and the 'Self-archiving Initiative'. There are two types of repositories: the thematic repositories with digital material that covers a particular discipline (e.g. repository "PANDEKTIS") and institutional repositories usually implemented and supported by an academic institution or research institution (e.g. "SUN" is the repository of the National Hellenic Research Foundation). Adding content into an institutional repository enables institutions and various social actors to manage and preserve it, and therefore maximize its value.