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This paper describes progress toward achieving high efficiency, multijunction solar cells for cost effective application in terrestrial PV concentrator systems. Small area triple junction GaInP/sub 2//GaAs/Ge solar cells have been fabricated with an efficiency of over 32% when measured by NREL under an AM1.5D spectrum at 47 suns concentration. Small changes to the device design can achieve similar efficiencies at concentration ratios of about 500 suns, resulting in cell costs of $0.5 to $0.6/W today, at production volumes of approximately 50 MW/year. This makes them highly cost effective in existing concentrator systems, compared to flat plate technologies. The future development of new 1 eV materials for space cells, in conjunction with further reduction in Ge wafer costs, promises to achieve solar cells of >40% efficiency that cost $0.4/W or less at these concentration ratios.