The FPS standard embodies the customary duty to provide protection and security to aliens. The obligation has been the subject of numerous political and legal disputes throughout the centuries. Most present-day investment treaties include an FPS clause. Still, the meaning and scope of the obligation continues to be the subject of intense controversy. Debate has chiefly focused on whether FPS is a guarantee of physical security only or ensures legal security too. In addition, FPS’s complex connection to the notion of due diligence poses significant challenges for theory and practice.