It is assumed that the channel is completely characterized by three factors: (a) its geometric shape, (b) the potential energy interaction between an ion and the channel wall, and (c) the potential energy interaction between two ions at arbitrary positions in the channel. The total potential energy of an ion in a multiion channel can be described by a summation over factors b and c. The ion-water interaction is described by a continuum diffusion coefficient which is determined by the channel geometry (c). Given this physical description, a theory is described that predicts the flux of all the ion species that are present, with no additional assumptions about, e.g., the maximum number of ions allowed in the channel, location of binding sites or shape of energy barriers. The solution is based on a combination of the Nernst-Planck and Poisson equation. The Poisson potential is corrected for the ion's self potential. A hard sphere ion-ion interaction is included that prevents ions from piling up on top of each other in regions where the channel wall has a high charge density. An exact analytical solution is derived for the region in the bulk solution, far from the channel mouth and this solution is used as a boundary condition for the numerical solution. The numerical solution is obtained by an interactive procedure that is surprisingly efficient. Application of the theory to the acetylcholine receptor channel is described in the companion paper (Levitt, D. G. 1990. Biophys. J. 59:278–288).