Abstract: Ion exchange membranes play a leading part in an electrodialyzer and the performance of an electrodialyzer strongly depend on the characteristics of ion exchange membranes. In order to operate an electrodialyzer effectively, it is necessary to understand the characteristics of ion exchange membranes definitely. This chapter describes the fundamental theories on the membrane characteristics based on the principles of electrochemistry. The origins of the theories are older than the invention of an ion exchange membrane itself and the reasonability of the theories had been confirmed by the experimental observations performed for a long time. With these investigations, the membrane characteristic measuring methods are established and have been applied widely. In this chapter, the measuring methods are explained definitely referring to that the membranes are applied to saline water desalination.