Structural studies on retinal proteins have advanced significantly in recent years. Among the proteins whose structure has been solved by X-ray crystallography, rhodopsin is the only one from eukaryotic organisms having visual function. The structural model of rhodopsin also represents the first atomic template for a much larger superfamily of G protein-coupled receptors. Its natural abundance in vertebrate retinal rod cells and a novel single-step purification method made it possible to obtain three-dimensional crystals for X-ray diffraction study. The ground state structure has been refined so far to 2.6 A resolution, by which the details of the hepta-helical transmembrane region are resolved including functional internal water molecules. Possible structural change upon photo-excitation is likely to involve interaction changes between retinal chromophore and the surrounding residues. Further studies with microspectroscopy and X-ray diffraction with improved crystals that diffract to 2 A resolution will ...