Power system oscillations without a big disturbance occur spontaneously in a power system and if they are not damped out properly may lead to grid failure. In this book we examine the methodology to study this phenomenon from several angles. Modeling the system to investigate these oscillations is given top priority along with physical interpretation of the phenomenon. The book covers low frequency 1-3 Hz as well as sub synchronous oscillations in the 10-50 Hz range. The latter are called torsional oscillations. Design of Power system stabilizers as well as damping techniques for sub synchronous oscillations are discussed. Modeling and design of FACTS devices is included. The small signal analysis of multimachine systems along with the selective computation of Eigen value(s) of interest in a large system is presented.