The engineering design of two high-power steadystate ECRH injection systems is presented for the MARS tandem mirror reactor. With a design power of 57 MW, System I is comprised of 1 MW cavity-mode gyrotrons coupled to a novel quasioptical launching system for the combination and transmission of the ECRH power to the plasma. System II has a design power of 84 MW and comprises 2.5 MW quasi-optical gyrotron units coupled to a quasi-optical launching system similar in principle to System I but displaying minimal space requirements. Potential operating conditions, parameters and constraints are presented for multi-MW gyrotrons and quasi-optical launching systems, and key ECRH development and technology needs for commercial tandem mirror reactors are defined.