Hi, That's a great question. Yes, a fully licensed version of ESXi is fine. However, I believe that there were some features that may have been disabled in the recently released free version that are currently being tested. The Double-Take 5.1 release supports ESX 3.5 and earlier versions with Virtual Center 2.5 and earlier. Double-Take usually doesn't care what the virtualization platform is as long as it runs a Windows operating system. However, there are some calls to the Virtual Center with the Virtual Recovery Assistant that are specific to VMware versions. Double-Take has been supporting VMware versions for years and don't expect the free version of ESXi will be an exception.