One of the biggest considerations for digital librarians is the need to provide long-term access to their resources; to do this, there are two issues requiring watchfulness: Media failure and format obsolescence. With media failure, a particular digital item is unusable because of some sort of error or problem. A scratched CD-Rom, for example, will not display its contents correctly, but another, unscratched disk will not have that problem. Format obsolescence is when a digital format has been superseded by newer technology, and so items in the old format are unreadable and unusable. Dealing with media failure is a reactive process, because something is done only when a problem presents itself. In contrast, format obsolescence is preparatory, because changes are anticipated and solutions are sought before there is a problem.