This book addresses two technological areas that have had profound impact on the design and construction of civil engineering structures under seismic loads. These areas are supplemental damping and seismic isolation. Research in and development of supplemental damping, or passive energy dissipation, for structural applications has a roughly 50-year history. However, only over the previous 20 years or so have we seen this technology mature to the point where innovative devices have been developed on the basis of sound principles and integrated into buildings and bridges—either as new construction or as retrofit—for seismic hazard mitigation. Seismic isolation as a research topic, in comparison with supplemental damping, has an older origin; and its operating principle is now well understood. In recent years, however, exciting progress in practice has been made in several directions; these include the development of new or improved isolation systems and the development of hybrid systems, namely, isolation systems in combination with supplemental damping devices. With this background, it is timely to see a book that collects and consolidates information from a diverse set of sources into an up-to-date unified presentation of both theoretical and practical aspects of these technologies. This book is divided into three main sections, supplemented by a Foreword by Professor V. V. Bertero, two sample seismic retrofit project assignments Chapter 11 , four appendixes listing structural implementations of these devices and systems dating up to about 2003 , and a comprehensive list of references. After brief introductory remarks, the first section Chapters 1–3 provides a concise review of the fundamentals of supplemental damping and seismic isolation, as well as current design philosophies from the viewpoint of performance-based earthquake engineering. Energy principles are introduced in Chapter 3, with a focus on energy as a design concept, as well as a common basis for performance comparison among various kinds of devices that have been studied and developed for structural applications. A comprehensive treatment of supplemental damping and supplemental damping devices dampers , the second main section of the book, is covered in Chapters 4–8. Chapter 4 presents basic concepts and general design and analysis procedures for structures with dampers; a large variety of dampers are introduced