Chalcopyrite copper indium sulfide (CuInS2) and copper indium gallium selenide (Cu(InxGa1-x)-Se-2; CIGS) nanocrystals ranging from similar to 5 to similar to 25 nm in diameter were synthesized by arrested precipitation in solution. The In/Ga ratio in the CIGS nanocrystals could be controlled by varying the In/Ga reactant ratio in the reaction, and the optical properties of the CuInS2 and CIGS nanocrystals correspond to those of the respective bulk materials. Using methods developed to produce uniform, crack-free micrometer-thick films, CuInSe2 nanocrystals were tested in prototype photovoltaic devices. As a proof-of-concept, the nanocrystal-based devices exhibited a reproducible photovoltaic response.