Optical mapping of the membrane potential with voltage-sensitive dyes is an advanced approach that involves in many theories such as molecular photonica, physiology and computer science. Compared with the conventional techniques of membrane patch-clamp and microelectrode, the optical mapping system can measure not only the changes of a single membrane potential from multiple sites simultaneously but also the conduction properties of the cell populations. So this technique provides an important method for studying the electrophysiology of the small cell, the tiny neurite and the cardiac arrhythmia, etc. Because this technique can also avoid the electromagnetic interferences, it uniquely provides an ideal means for studying the mechanisms such as cardiac defibrillation. In this paper the principles and the system structure of the optical mapping are introduced, its applications and future developments are also presented at the same time.