The events of September 11 may present appraisers with a unique valuation challenge related to buildings occupied by government agencies pursuant to a lease with a private owner. As federal, state, and local governments respond by increasing security measures in buildings housing their agencies, they may exert control over the exterior and interior common areas in ways not contemplated by the lease. Unless compensated through bilateral negotiations with the landlord, a government's increased security activities in the common areas may trigger liability under the Takings Clause of the United States Constitution's Fifth Amendment. The purpose of this article is to demonstrate how a government's unilaterally imposed security measures in the common areas of a privately owned multi-tenant office building can constitute a "taking" of property requiring just compensation under the Constitution and to set forth the legal principles applicable to the determination of just compensation. While the issues discussed in this article may be relevant to offices leased by state and local governments, we focus on office space leased by the federal government because of its nationwide prevalence.