Heptahelical G protein coupled receptors (GPCRs) reside in cell membranes and couple to heterotrimeric guanine nucleotide-binding proteins (G proteins) to relay extracellular signals to intracellular signaling networks. These receptors represent the largest family of membrane proteins in our body. Accordingly, GPCRs are involved in the control of virtually all physiological processes and due to their importance in human health are among the top drug targets. Despite decades of research on the physiology of GPCRs, the mechanism of signal transfer to the G protein is still not well understood. Here we provide a mini review on this topic by focusing on rhodopsin and transducin (Gt), the GPCR and cognate G protein of photoreceptor cells in the vertebrate eye. The latest information about structural studies of rhodopsin and transducin, as well as current models for the universal coupling mechanism are discussed.