CuInSe2 and its related I-III-VI2 chalcopyrite compounds have been studied for about 50 years for application to absorbers in polycrystalline thin-film solar cells. Several research groups developed Cu(Ga,In)Se2, Cu(Ga,In)(S,Se)2, and (Cu,Ag)(Ga,In)Se2 solar cells with conversion efficiencies higher than 20%. The bandgap energies of these absorber materials in high-efficiency solar cells are less than 1.2 eV. Currently, wide bandgap chalcopyrite compounds, such as Ga-rich Cu(Ga,In)Se2, Cu(Ga,In)S2, and Ga-rich (Cu,Ag)(Ga,In)Se2, have gained attention as absorber materials for the top cells in tandem structure solar cells. However, the conversion efficiency of these wide bandgap solar cells cannot reach 20%. I-III-VI2 chalcopyrite compounds are composed of three or more elements and their crystal and electronic structures are more complicated than III-V or II-VI compound semiconductors. In particular, phase diagrams containing the I-III-VI2 chalcopyrite compounds are complex and vary greatly depending on the material systems. This article provides an overview of the current state of our materials science understanding of I-III-VI2 chalcopyrite compounds. In particular, we discuss the differences between CuInSe2 and the other chalcopyrite compounds based on their phase diagrams of I2VI-III2VI3 pseudobinary systems such as Cu2Se-In2Se3.