The three-dimensional structure of the photosynthetic reaction center from Rhodobacter sphaeroides is described. The reaction center is a transmembrane protein that converts light into chemical energy. The protein has three subunits: L, M, and H. The mostly helical L and M subunits provide the scaffolding and the finely tuned environment in which the chromophores carry out electron transfer. The details of the protein-chromophore interactions are from studies of a trigonal crystal form that diffracted to 2.65-A resolution. Functional studies of the multi-subunit complex by site-specific replacement of key amino acid residues are summarized in the context of the molecular structure.