[en] The TITAN reversed-field-pinch (RFP) fusion-reactor study aims to determine the technical feasibility and key developmental issues for an RFP fusion reactor operating at high power density; and to determine the potential economic, operational, safety and environmental features of high mass-power-density fusion-reactor systems. Mass power density (MPD) is defined as the ratio of net electric output to the mass of the fusion power core (FPC). The FPC includes the plasma chamber, first wall, blanket, shield, magnets, and related structure. Two different detailed designs TITAN-I and TITAN-II, have been produced. TITAN-I is a self-cooled lithium design with a vanadium-alloy structure. TITAN-II is a self-cooled aqueous loop-in-pool design with 9-C ferritic steel as the structural material. Both designs use RFP plasmas operating with essentially the same parameters. Both conceptual reactors are based on the DT fuel cycle, have a net electric output of about 1000 MWe, are compact, and have a high MPD of 800 kWe per tonne of FPC. The inherent physical characteristics of the RFP confinement concept make possible compact fusion reactors with such a high MPD. The TITAN designs would meet the U.S. criteria for the near-surface disposal of radioactive waste (Class C, 10CFR61) and would achieve a high Level of Safety Assurance with respect to FPC damage by decay afterheat and radioactivity release caused by accidents. Very importantly, a 'single-piece' FPC maintenance appears feasible for both designs. The design window for such compact RFP reactors would include machines with neutron wall loadings in the range of 10-20 MW/m² with a shallow minimum COE at about 18 MW/m². Even though operation at the lower end of this range of wall loading (10-12 MW/m²) is possible, and may be preferable, the TITAN study adopted the design point at the upper end (18 MW/m²) in order to quantify and assess the technical feasibility and physics limits for such high-MPD reactors. From this work, key physics and engineering issues central to achieving reactors with the features of TITAN-I and TITAN-II have emerged. (orig.)