Transient stability of electric energy grids is defined as the ability of the power system to remain in synchronism during large disturbances. If the grid is not equipped with controllers capable of transiently stabilizing system dynamics, large disturbances could cause protection to trigger disconnecting the equipment and leading further to cascading system-wide blackouts. Today’s practice of tuning controllers generally does not guarantee a transiently stable response because it does not use a model for representing system-wide dynamic interactions. To overcome this problem, in this thesis we propose a new systems modeling and control design for provable transient stabilization of power systems against a given set of disturbances. Of particular interest are fast power-electronically-controlled Flexible Alternating Current Transmission System (FACTS) devices which have become a new major option for achieving transient stabilization. The first major contribution of this thesis is a framework for modeling of general interconnected power systems for very fast transient stabilization using FACTS devices. We recognize that a dynamic model for transient stabilization of power systems has to capture fast electromagnetic dynamics of the transmission grid and FACTS, in addition to the commonly-modeled generator dynamics. To meet this need, a nonlinear dynamic model of general interconnected electric power systems is derived using time-varying phasors associated with states of all dynamic components. The second major contribution of this thesis is a two-level approach to modeling and control which exploits the unique network structure and enables preserving only relevant dynamics in the nonlinear system model. This approach is fundamentally based on separating: a) internal dynamics model for ensuring stable local response of components; b) system-level model in terms of interaction variables for ensuring stability of the system when the components are interconnected. The two levels can be controlled separately which minimizes the need for communication between controllers. Both distributed and cooperative ectropy-based controllers are proposed to control the interaction-level of system dynamics. Proof of concept simulations are presented to illustrate and compare the promising performance of the derived controllers. Some of the most advanced FACTS industry installations are modeled and further generalized using our approach.