Digital permanence addresses the history and development of digital storage techniques, specifically quantifying the expected lifetime of data stored on various digital media and the factors which influence the permanence of digital data. It is often a mix of ensuring the data itself can be retained on a particular form of media and that the technology remains viable. Where possible, as well as describing expected lifetimes, factors affecting data retention will be detailed, including potential technology issues. Since the inception of computers, a key concept differentiating computers from other calculating machines has been their ability to store information. Over the years, various hardware devices have been designed to store ever larger quantities of data. With the development of the Internet the quantity of information available appears to continue to grow at an ever-increasing rate often characterised as an information explosion. As information stored on traditional media such as hand-written documents, printed books, photographic images and the likes is being replaced by digital files, so humanity's social and cultural legacy to future generations will depend more and more on the permanence of digital information. However, not all this information is worth saving for any length of time; sometimes its value can be very short-lived. Other data, such as legal contracts, literature, scientific studies, are frequently expected to last for centuries. This article describes how reliable different types of storage media are at storing data over time and factors affecting this reliability. Librarians and archivists responsible for large repositories of information take a deeper view of electronic archives. Data format Data must be stored in a format which can be meaningfully accessed now and in the future. Technology reliance If data requires a special program to view it, say, as an image, then software must also be available to both interpret the basic data file and also render it appropriately. In some cases, this might also require special hardware. Archival strategy Data must remain available in the long term. At present, a growing problem is the time taken to reproduce an archive, for instance following a hardware or system upgrade. Since the sheer volume of archive data continues to grow, new hardware is always required to maintain the archive and so regular migration of data to a new system must be performed on a regular basis. The time taken to migrate data is starting to approach the frequency of system upgrade, such