Tier 0 is the highest-trust administrative layer in Microsoft identity environments. It includes the systems that control the entire security boundary, such as domain controllers, identity servers, and other crown-jewel assets that can grant or revoke access across the network.
This tier matters because compromise here usually means full domain control. If an attacker steals credentials from a Tier 0 admin or reaches a Tier 0 host, they may be able to change group memberships, reset privileged passwords, or extract directory secrets. Defenders therefore isolate Tier 0 with strict segmentation, separate admin accounts, privileged access workstations, and minimal standing privilege. In real attacks, Tier 0 is often the final target after initial access, because controlling it turns a local foothold into organization-wide authority.