This book deals with the application of new techniques based on multivariable control theory and optimisation theory to the study of robust stability of highly uncertain models of large interconnected power systems subject to real parameter variations. It focuses on the study of robust stability problems associated with parameter variations representing real physical quantities. The objective is to verify that critical system controllers of complex systems remain stable and achieve desired performance objectives for all predefined power system variations at selected operating conditions along its expected operating trajectory. The second related objective is to determine the stability robustness with respect to changes in power system parameters and the maximum loading condition for which the system will remain stable.