III-V tandem solar cells can absorb light across a broad range of wavelengths in the solar spectrum and are ideally suited to efficiently convert UV, visible and near infrared photons into electricity. In combination with silicon as a bottom cell, highly efficient tandem solar cells with two or even three sub-cells can be realized. The III-V sub-cells are only a few micrometers thick, due to the high absorption properties of the III-V compound semiconductor materials. This combination of materials opens up interesting ways for a cost-effective production, however, there are still challenges to be met, which we are addressing in our ongoing R&D projects: One challenge is to develop a cost-effective epitaxial growth of III-V layers on silicon, i.e. to overcome the lattice constant differences in the various materials. Currently we are investigating two concepts for 2-terminal monolithic solar cells. On the one hand, the III-V layers are grown directly on silicon, and on the other hand, they are transferred from a GaAs to Si substrate using wafer or adhesive bonding. At Fraunhofer ISE we achieved a peak conversion efficiency of 34.5% (AM 1.5g) for a III-V silicon tandem solar cell, thus establishing a new world record.