This chapter contains sections titled: Introduction Classification of G Protein-coupled Receptors The Extracellular N-terminal Domain of G Protein-coupled Receptors Sequence Analyses of the 7TM Segments of the Rhodopsin Family of G Protein-coupled Receptors The Conformation of Pro-kinked Transmembrane α-Helices Helix Deformation in the Rhodopsin Family of G Protein-coupled Receptors Transmembrane Helix 1 Transmembrane Helix 2 Transmembrane Helix 3 Transmembrane Helix 4 Transmembrane Helix 5 Transmembrane Helix 6 Transmembrane Helix 7 Structural and Functional Role of Internal Water Molecules A Conserved Hydrogen Bond Network Linking D2.50 and W6.48 The Environment of the NPxxY Motif in TM7 Molecular Processes of Receptor Activation Molecular Processes Initiated by the Recognition of the Extracellular Ligand by the Receptor Molecular Processes that Propagate the Signal from the Ligand Binding Site to the Intracellular Amino Acids of the Transmembrane Bundle Conclusions