This is one of the first texts in which electro-diffusion of ions in its different aspects is considered as a unified subject. As a subject it is relevant to understanding the behavior of apparently different physical objects such as electrolyte solution, ion-exchangers, ion-selective membranes, and semiconductors. This text treats a selection of topics in electro-diffusion of ions in an aqueous medium --- a nonlinear transport process whose essence is diffusion of ions combined with their migration in a selfconsistent electric field. Electro-Diffusion of Ions is intended to enhance the understanding of particular electro-diffusional transport phenomena and to spark the interest of the mathematical community in the corresponding area of applications and development of a unified scientific vision. Contains some previously unpublished results for electro-diffusional problems.