The construction and operation of ionomer-film modified electrodes are described, including the fundamentals of the ion-exchange processes. Ion-exchange selectivity coefficients for a series of alkylsubstituted pyridinium cations in Nafion have been measured and the large values of these coefficients are attributed to the hydrophobic effect; a free-energy relationship relating the coefficient to the size of the ion has been established. The use of Nafion-coated electrodes for the in vivo measurement of neurotransmitters and for the preconcentration of analytes present in low concentration is discussed. Experiments on the dynamics of the response of ionomer modified electrodes are reported and very high ionic transport rates can be achieved for novel ionically conductive composite polymer membranes. The covalent attachment of redox groups such as ferrocene allows one to control the release of counterions by switching the redox state of the ferrocene.