The Smart Grid will incorporate computer networking technologies into the electrical generation, transmission, and distribution sectors. Thus, there will be an underlying Critical Information Infrastructure (CII) based on these network connections. This CII is vulnerable to traditional cyber or computer based attacks typically geared toward disabling devices or networks. However, the Smart Grid is also vulnerable to physical attacks where sensors are tricked into reporting false conditions that cause the control system to react in an inappropriate manner. Cyber-physical attacks blending both cyber and physical attack components are also a possibility. Techniques to model cyber-attacks exist, and this chapter presents a modeling methodology, termed hybrid attack graphs, to model cyber-physical attacks. The hybrid attack graph formalism can be applied to develop best practice guidelines and security patches for the Smart Grid. This formalism can also be applied to other cyber-physical domains as well to help bridge the gap between the physical, logical, and network domains.