The direct-current potential difference method (DC-PDM) is known as one of the best techniques to evaluate the damage initiated in structural materials. However, it is not still very popular and has not yet been widely used. One of its main reasons is that the potential difference measured is not supported based on the physical background and the relationship between the damage and the potential difference is not clarified quantitatively. In this paper, DC-PDM is classified into several categories based on the distribution of defects, the number of potential measurement probes, and the location of current supply probes. Characteristics of all categories are described.