This book is a systematic and comprehensive exposition of the state-of-the-art research results in the literature on equilibrium problems. The book describes the highest-level research and reflects a current picture of results in the literature on the three topics in a very central place of the general theory of equilibrium problems: existence, stability, and approximation, together with their particular cases, and numerous applications. It is intended to serve as both introductory and deep courses for graduate students; or as useful materials for researchers studying aspects of optimization and equilibrium problems or, more generally, working on inter-discipline such as mathematical economics, operations research and management, or even in various areas of science and technology. In providing profound knowledge of recent research, this book has advantages over existing recent books focused on equilibrium problems and variational relation problems and may also be suitable for readers preparing deep and comprehensive graduate courses.