Muscarinic acetylcholine receptors play important roles in numerous physiological functions including higher cognitive processes such as memory and learning. Consistent with the well-documented pharmacological heterogeneity of muscarinic receptors, molecular cloning studies have revealed the existence of five distinct muscarinic receptor proteins (M1-M5). Structure-function relationship studies of the cloned receptors have been greatly aided by the high degree of structural homology that muscarinic receptors share with other G protein-coupled receptors. In this review, Jürgen Wess discusses recent mutagenesis studies that have considerably advanced our knowledge of the molecular details underlying muscarinic receptor function.