This Chapter considers the problem of identifying strategic substations and transmission lines in a power system from the point of view of system security and interconnection integrity. Substations are considered critical for system security not from the traditional consideration of n − 1 contingencies but as nodes in an interconnected network disconnection of which due to severe disturbances may cause catastrophic consequences to the system. The concept of network centrality is applied to rank substations according to their structural role in the system and spectral graph theory concepts are used to identify node centrality. The new centrality measures are based solely on the analysis of the structure of the electric power network and the required data can be readily found in the raw data file for the power flow programs used by the electric utilities. Strategic transmission lines are defined in this Chapter as those whose removal could separate the system in a partition such that a severe disturbance effects could be mitigated. Those transmission lines are identifying also by using concepts of spectral graph theory as explained in this Chapter.