This paper addresses the modeling and closed-loop control of large-scale electric power systems operating under normal conditions. A new structure-based modeling approach is introduced-an approach which reveals fundamental properties of power system dynamics, and offers physical insight into the interactions between its constituent subsystems. These properties and insights lead to the formulation of simple, higher level models which represent slow interactions among subsystems driven by slow load variations. The models which evolve from this structure-based approach enable hierarchical control design, without any a priori assumptions regarding the strength of the interactions among the subsystems. New control designs are thus proposed at both the subsystem and the interconnected system levels, a key element in heir formulation being that both react to variables reflecting interactions between subsystems. Reactive power/voltage (Q/V) control problems are used to illustrate the proposed modeling and control approach.