The book is devoted to out-of-step (OOS) protection systems. It consists of two parts. Chapters 1–6 are dedicated to the problem of angular stability and OOS relaying principles. Traditional protection systems as well as potential prospective implementations are considered in these chapters. An outline of the most recent achievements in OOS relaying philosophy is provided. A new algorithm for generator grouping and OOS protection is presented. Chapters 7 and 8 review the existing and emerging communication technologies and communication standards that are already in use in power systems and can be used in the future to improve the performance of OOS protection systems.