Book excerpt: This book details the state of the art in the development and application of the transient energy function (TEF) method as a tool for power system transient stability assessment. It provides both the analytical foundations of the TEF method and the practical issues involved in the application of the method to analyze power systems. Of primary interest to electric utility engineers who need to understand and apply the technique, as well as engineers in research organizations involved in research and development projects on power system dynamics, and utility engineers interested in the use of the TEF method as a tool for dynamic security assessment.