With the competitive market environment in which power systems at present operate, the need for optimal system operation and at the same time maintaining system security is on the increase, and represents a challenge to system operators. With this acknowledgement, the book has first the focus on the steady-state models of a power system together with its elements, including FACTS devices such as SVC, STATCOM and UPFC. The comprehensive steady-state model of power systems with FACTS devices provides the foundation for the second part of the book in the field of secondary control associated with the restoration of power system security for both voltage and power-flow aspects, following a disturbance or contingency. The control schemes based on constrained optimisation techniques inherently coordinate optimally the controllers and also lead to maximum network operational benefits. The book is a valuable reference for both undergraduate and postgraduate students, engineers, and researchers in relation to planning, design and operation of power systems.