- Multi-user operating systems allow multiple users to access and use the system concurrently. - UNIX, Windows 2000, and even later versions like Windows XP (though primarily desktop OS, they have features like Fast User Switching and support for services running under different users, and Windows 2000 was a significant multi-user capable OS designed for servers/workstations) are capable of multi-user operation to varying extents. - MS-DOS strictly lacks multi-user capabilities.