Our collaborative review article about semiconductor alloy nanomaterials is published in Nature Reviews Materials. Over the past decade there has been tremendous progress in the development of nanoscale semiconductor materials with a wide range of bandgaps, obtained by alloying different individual semiconductors. One important common feature of these materials is that their nanoscale dimensions result in a large tolerance to lattice mismatches within a monolithic structure of varying compositions or between substrate and target material, which allows the achievement of almost arbitrarily controlled variations of the alloy composition. As a result, the bandgaps of these alloys can be tuned widely without the detrimental defects that are often unavoidable in bulk materials, which have a much more limited tolerance to lattice mismatches. This class of nanomaterials could have a far-reaching impact on a wide range of photonic applications including tunable lasers, solid state lighting, artificial photosynthesis and new solar cells