The electric power grid is a complex cyber-physical system, whose reliable and secure operation is of paramount importance to national security and economic vitality. There is a growing and evolving threat of cyber-based attacks, both in numbers and sophistication, on the nation's critical infrastructure. Therefore, cyber security "encompassing attack prevention, detection, mitigation, and resilience" is critical in today's power grid and the emerging smart grid. The goal of this project is to develop a unified system-theoretic framework and analytical tools for cyber-physical security of power systems, capturing the dynamics of the physical system as well as that of the cyber system. Research tasks include: 1) Development of a methodology for impact analysis that includes systematic identification of worst-case stealthy attacks on the power system's wide-area control and evaluating the resulting consequences in terms of stability violations and performance loss. 2) Development of robust cyber-physical countermeasures, employing a combination of methods from system theory, cyber security, and model-based/data-driven tools, in the form of domain-specific anomaly detection/tolerance algorithms and attack-resilient control algorithms. 3) Evaluating the effectiveness of the proposed impact modeling and mitigation algorithms through a combination of simulation and testbed-based evaluations, using realistic system topologies and attack scenarios. The project makes significant contributions to enhance the security and resiliency of the power grid and lays a scientific foundation for cyber-physical security of critical infrastructure. Also, the project develops novel curriculum modules, mentors graduate and undergraduate students including under-represented minorities, leverages industrial collaborations, and exposes high school students to cyber security concepts.