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A commercial lattice-matched InGaP/InGaAs/Ge solar cell has reached an average efficiency of 40% at 500 kW/m2. The design changes that lead to this result are discussed. These data are complemented with a presentation of the latest new solar cell development results from the laboratory. Inverted metamorphic multijunction (IMM) solar cells have been prototyped with 42.4% efficiency at 325 suns for concentrator applications and 33.6% efficiency at 1 sun AM0 for space applications. Six subcell devices are now under development. These results are used, along with other experimental data and other industrial constraints, as input to a computer model to predict what practical efficiency might be achievable with this device approach. The computer model suggests that 45% and 50% efficiencies are technologically feasible with a three-junction and five-junction device, respectively, at an irradiance of 500 kW/m2 and 25 °C using known materials, device architectures, and manufacturing methods.