The recent resolution of G-protein-coupled receptor (GPCR) structures in complex with Na(+) bound to an allosteric modulatory site has renewed interest of the regulation of GPCRs by ions. Here, we summarise key data on ion modulation of GPCRs, obtained in pharmacological, crystallographic, mutagenesis and molecular modelling studies. We show that ion modulation is a highly complex process, involving not only cations but also, rather neglected until now, anions. Pharmacotherapeutic and toxicological aspects are discussed. We provide a mathematical framework for the analysis of ion effects. Finally, we discuss open questions in the field and future research directions. Most importantly, the in vivo relevance of the modulation of GPCR function by monovalent ions must be clarified.