For research libraries, the long-term preservation of digital collections may well be the most important issue in digital libraries. In certain ways, digital materials are incredibly fragile, dependent for their continued utility upon technologies that undergo rapid and continual change. In the world of physical research materials, a great number of valuable research resources have been saved passively: acquired by individuals or organizations and stored in little-visited recesses. These physical resources are still viable decades later. This is not the case with the digital equivalents. Changes in computing technology will insure that, over relatively short timeframes, both the media and the technical format of old digital materials will become unusable. Keeping digital resources accessible for use by future generations will require conscious effort and continual investment.