The band gap of the quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single-junction strain-balanced quantum well solar cell (SB-QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB-QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain-balanced quantum wells in multi-junction solar cells can current match the sub-cells without the introduction of dislocations. Calculations are shown which predict efficiency limits as a function of QW absorption and band gap for such cells. A dual-junction InGaP/GaAs solar cell with QWs in the bottom sub-cell has been grown and characterized. Laboratory and calculated efficiencies relative to control cells are presented for the reported cell and a modeled device, respectively. Copyright © 2011 John Wiley & Sons, Ltd.