This book provides trends for the development of novel membranes as separators with applicable properties. The authors offer a comprehensive review of the various types of polymeric materials used as separators either as an electrolyte or not, in different kinds of batteries. The mathematical formulations of both membrane and fluid parts are reviewed, and general governing equations of the membrane are presented. Also, general formulations of the fluid region and interface conditions between the membrane and fluid sides are presented. The concluding chapter focuses on the estimation of the plasma membrane potential in yeast by the fluorescence changes of various indicators. The most used of these indicators have been DiSC3(3) and DiSC3(5).