The previous editions of Power System Relaying are used as a textbook by universities and in industry courses throughout the world. The theory and fundamentals of relaying constitute the major part of the book. The third edition covered specific relaying concepts and circuits, as well as major system devices such as current and potential transformers and transmission lines. The fourth edition provides the reader with essential updates. References are revised throughout. Chapter One includes modern coverage of circuit breakers. With the increasing use of computer relays the application of relays with mho-type configuration has changed dramatically. Chapter Five has been added to, with more information on relays for multi-terminal lines. In recent years, the new concept of the smart grid has introduced ideas and practices that are changing the design, operation and performance of the distribution system. Essentially, the impact on system and bus configurations, including the smart-grid concept is discussed in more detail in Chapter Six. There is now a distribution smart grid diagram showing added generation devices. The current differential summary has been updated. According to the Final Report of the August 14, 2003 Blackout in the United States and Canada, the societal cost of a blackout is estimated to be around $10 billion per event. Chapter Ten on Bus Protection Guide IEEE C37.234 (2009) has been revised, along with the section on restoration. The problem of mixing analog and digital within a common overall protection system and the lack of standardization between manufacturers is addressed and discussed in Chapter Thirteen. Chapter Fourteen fully explains improved protection with WAMS. It describes organization, how to use WAMS for protection, and SIPS and control with WAMS. The final chapter is entirely new and covers protection considerations for renewable energy sources. It looks at types of renewable energy sources, grid interconnection techniques and codes, and protection considerations and practices.