The secure information sharing problem is one of the oldest and most fundamental and elusive problems in information security. Mission objectives dictate that Information must be shared and made available to authorized recipients, and yet information must be protected from leakage and subversion by malicious insiders and malicious software. The doctrine of "share but protect" indicates the inherent conflict in achieving effective secure information sharing. In this talk we demonstrate the complexity and richness of the secure information sharing problem space. We then identify some "sweet spots" that appear promising in their practical benefit and feasibility of solutions. We describe the PEI models approach to decompose security problems into the three layers of policy models (topmost), enforcement models (middle), and implementation models (bottom). We discuss how this approach can be applied to the secure information sharing problem. Finally we indicate how modern trusted computing technology can be used to solve important variations of this problem.