A striking level of diversity of visual systems in different species reflects their adaptive responses to various light environments. To study the adaptive evolution of visual systems, we need to understand how visual pigments, the light-sensitive molecules, have tuned their wavelengths of light absorption. The molecular basis of spectral tuning in visual pigments, a central unsolved problem in phototransduction, can be understood only by studying how different species have adapted to various light environments. Certain amino acid replacements at 30 residues explain some dim-light and color vision in vertebrates. To better understand the molecular and functional adaptations of visual pigments, we must identify all critical amino acid replacements that are involved in the spectral tuning and elucidate the effects of their interactions on the spectral shifts.