During the past decades, employment of supplementary systems for seismic control of structures has gained increasing acceptance, and research has flourished on development and performance characterization of such systems. These include isolation systems and energy dissipation devices (dampers). This study is devoted to development of a new hysteretic damper for seismic protection of buildings. The new system is named “Torsional Hysteretic Damper” (THD). THD, is a mechanical device designed to utilize torsional yielding of cylinders made of ductile steel to dissipate the kinetic energy associated with imposed differential movements. The newly-developed damper is designed for deployment in building frames using inverted-V braces. The scope of this research includes all of the developmental stages of THD, namely, conceptual development, analytical and numerical studies and verification tests. This report is the third and the final report of the project and contains detailed information on all phases of the research including the developmental stages (Chapter 1-2) of the new damper and analytical and experimental studies. Details of analytical studies which were performed to establish the necessary equations for force-displacement analysis of the new damper and its components are presented in this report (Chapter 3 to 5). In the second part of the report, the experimental studies which were performed on three scaled frame specimens are described. Detailed design of the test specimens are given in Chapter 6. The frames are an inverted-V braced (Chevron) frame, a H-link eccentrically braced frame and a frame equipped with THD damper. Chapters 7 presents a description of the test setup and the preparatory steps taken prior to the execution of the tests. In Chapter 8, the observations from the tests are reported and the results from the tests are presented and discussed. In the final stage of this research a computer program has been developed to allow for easy design of the components of the THD. In Chapter 9 of this report a description of the design algorithm is given and the developed program is explored. In the last chapter (Chapter 10) of this report, summary and conclusions are presented