G protein-coupled receptors (GPCRs) are a large superfamily of membrane receptors that have seven transmembrane helices and respond to a wide array of signaling ligands. Similarities in the sequences of these receptors have led to the idea that they share a common activation mechanism. In contrast, sequence diversity between receptors has been correlated with their specificity for different ligands. I will present recent results on GPCRs from three different subfamilies: rhodopsin from the visual receptor subfamily, the b2-adrenergic receptor from the amine receptor subfamily and CCR5 from the peptide receptor subfamily. The structural changes within these three GPCRs provide a framework for understanding how GPCRs are activated.