What is the difference between Zero Trust and perimeter security?

Traditional perimeter security assumes that users and devices inside the corporate network are relatively trustworthy, while traffic from outside is treated as higher risk. Zero Trust works differently: every access request is evaluated continuously based on identity, device health, policy, risk, and the sensitivity of the resource being accessed, regardless of location. In modern Microsoft 365 environments, where users work remotely and data moves across cloud apps and external collaboration channels, that shift from implicit trust to continuous verification is the core difference.