Bacteriorhodopsin (bR) and halorhodopsin (hR) found in the cell membrane of halobacteria are the simplest known light energy-converting systems. They are small proteins and require only light and a lipid bilayer to generate an electrochemical ion gradient. In overall structure, chromophore, and photoreactions, they closely resemble the visual pigments of animals, but those are signal, not energy, transducers and no obvious sequence homologies originally indicated an evolutionary connection. Signal-transducing archaebacterial rhodopsins have also been detected, but they are closely related to the energy transducers. These pigments show the same seven-helix structural motif common to energy-transducing bacterial rhodopsins, visual pigments, and other G-protein-linked sensors. Nevertheless, their sensory function is not mediated by either ion transport or G-proteins, but resembles the chemosensory transduction system of eubacteria. This review, however, will trace only the development of a mechanistic model for the active transport function of bR up to 1995.