Modern systems such as the electric smart grid consist of both cyber and physical components that must work together; these are called cyber-physical systems, or CPS. Securing such systems goes beyond just cyber security or physical security into cyber-physical security. While the threats multiply within a CPS, physical aspects also can reduce the threat space. Unlike purely cyber systems, such as the internet, CPS are grounded in physical reality. In this project, this physical reality is used to limit an attacker's ability to disrupt the system by limiting his/her ability to lie about his/her actions; if an attacker is inconsistent with physical reality, his/her actions are detectable and damage his/her reputation for future interactions with the system. The impacts of this work are far-reaching, as it creates a basis for developing inherently security CPS for not only the electric smart grid, but also advanced transportation and building environmental systems. A new generation of interdisciplinary scientists and engineers are being trained through this research. This project formulates a novel methodology that incorporates knowledge from both the cyber and physical domains into a distributed algorithm and ensures the trustworthiness, thus security, of the composed system. Metrics for security are also derived and rest on logical invariants that express correctness. The invariants either check the validity of a local action or the accuracy of remote data. They may be used as guards against an action, or may be incorporated into a dynamic reputation-based algorithm. As a testbed, a multilateral energy system on an electrical network will be studied. Preliminary studies of this system have resulted in algorithms that isolate malicious nodes within the context of a single algorithm, using a reputation metric that compares cyber information flows to physically measurable signals. The work will be extended to other algorithms and other related power systems, a generalizable framework will be developed, and more complete metrics will be derived. The project has important broader impact. It develops new approaches for securing critical infrastructure based on both and cyber and physical system aspects. The project also includes graduate and undergraduate involvement in cyber-physical systems research and design through involvement with testbeds and the Missouri Science and Technology Solar House team which designs and constructs houses for competition in the US Department of Energy Solar Decathlon.