III-V multi-junction solar cells have become standard in space and in terrestrial concentrator systems. These devices reach the highest efficiencies of any photovoltaic technology so far. This is enabled by stacking solar cells of several III-V compound semiconductors, which absorb different parts of the solar spectrum. The current landmark for III-V multi-junction solar cells is the Ga0.50In0.50P/Ga0.99In0.01As/Ge triple-junction, which only contains lattice-matched layers. This MOVPE-grown device is available from different suppliers in large quantities with efficiencies around 40% under concentrated sunlight. A record efficiency of 41.6% has also been achieved. Due to the mature development status of this device new concepts are being investigated to increase the efficiencies further. A toolbox of new ideas and methods is available such as metamorphic and/or inverted growth, wafer-bonding, different substrates, nanostructures, MBE growth and new materials. These have been used and combined intensively in recent years to develop and realize new structures of III-V multi-junction solar cells. The aim of this paper is to give an introductory overview about the status and the research trends in III-V multi-junction solar cells.