The Guide Specifications for Seismic Isolation Design, 3rd Edition, addresses major changes in the way seismic hazard is now defined in the United States, as well as changes in the state of the art of seismic isolation design for highway bridges. It also reflects changes in the definition of the seismic hazard as now defined in the AASHTO LRFD Bridge Design Specifications and the Guide Specifications for LRFD Seismic Bridge Design, industry trends in the design and construction of isolators, and provisions in the design specifications that impact the design and testing of isolation bearings.