Virtual tape library (VTL) technology is arguably the easiest way to introduce disk into a tapebased backup infrastructure. A VTL is a set of disks that emulate tape; you see a disk array, your backup software sees a tape drive or a tape library. The use of tape virtualization will improve backup performance and reliability while allowing you to preserve investment in your existing tape infrastructure. But choosing the right VTL technology depends upon what particular backup issues are most important in your environment. This paper considers these issues, then provides a quick review of Sun Microsystems' tape virtualization solutions, identifying how their offerings can best be leveraged to produce an optimized backup infrastructure.