Ionophores are lipophilic complexing agents capable of reversibly binding ions and catalyze ion transport across hydrophobic membranes. Therefore ionophores are key components of sensors, like ion-selective electrodes (ISE) or optodes. In their uncomplexed (or unassociated) form ionophores are either charged or electrically neutral. The first neutral ionophores used in ISE membranes were antibiotics. They were followed by a large number of natural and synthetic, mainly uncharged carriers for cations and a series of charged and uncharged ones for anions.