This chapter presents a novel model to assess the interdependencies between electric power systems interconnected with natural gas systems. The impact from natural gas systems in the electric power system can be evaluated with the proposed model in normal operation and contingency situations. To reduce the impact of interdependencies, additional constraints to the optimal dispatch problem are formulated. The interdependency constraints can be integrated into the normal optimal power flow problem and security-constrained optimal power flow problem to improve the robustness of the electric power system. A co-simulation platform is built in MATLAB environment. We evaluate the proposed model using the IEEE 14-bus system and a corresponding natural gas transmission system. According to the simulation results, the reliability of the power system is improved when interdependency constraints are considered.