Following the molecular cloning of five distinct muscarinic acetylcholine receptor (mAChR) genes, the last decade has witnessed an explosion of new knowledge about how mAChRs function at a molecular level. These studies have been greatly facilitated by the molecular characterization of the many components of the signal transduction pathways activated upon mAChR stimulation. Molecular genetic and biochemical approaches have considerably advanced our knowledge about how mAChRs are assembled, how they bind ligands, and which structural elements on the mAChRs are critical for G protein coupling. In addition, the molecular mechanisms involved in the regulation of mAChR activity (including mAChR sequestration, down-regulation, and phosphorylation) have been explored in great detail. Since the mAChRs are typical members of the superfamily of G protein-coupled receptors, the information gathered with this class of receptors should be of broad general relevance.