Modern electric power systems have increased the usage of switching power converters. These tightly regulated switching power converters be have as constant power loads (CPLs). They exhibit a negative incremental impedance in small signal analysis. This negative imped-ance degrades the stability margin of the interaction between CPLs and their feeders, which is known as the negative impedance instability problem. The feeder can bean LC input filter or an up stream switching converter.Active damping methods are preferred for the stabilization of the system. This is due to their higher power efficiency over passive damping methods. Based on different sources of damping effect, this paper summarizes and classifies existing active damping methods into three categories. The paper further analyzes and compares the advantages and disadvantages of each active damping method.