A theoretical description of the steady-state potential response of ionophore-based ion-selective electrodes is presented that, so far, is the most general formalism available. The treatment considers membrane systems with any number of ionophores, differently charged cations, anions, and fixed or stationary ionic sites. The theory accounts for thermodynamically controlled selectivity characteristics, as well as for various diffusion-induced effects resulting from transmembrane ion fluxes at zero current. The phenomena discussed in detail include apparent super- and sub-Nernstian responses and detection limits. An extension of the treatment for time-dependent phenomena is also given. The present approach can be applied for optimizing the selectivity coefficients and improving the detection limits of ionophore-based ion-selective electrodes.