On the surface, Microsoft's decision to replace the Exchange Server 5.5 Directory Store with Windows 2000 Active Directory (AD) might seem risky. (For more information about Exchange Server's switch from the Directory Store to AD, see "Exchange 2000 Server and Active Directory," December 1999.) After all, the Directory Store has worked well since 1996, so why move to a new directory that forces an upgrade of the underlying OS? Table A shows the advantages that Exchange 2000 Server, using AD, gains over Exchange Server 5.5.