When two or more signals of different frequencies pass through a nonlinear system, intermodulation distortion (IMD) occurs, resulting in the formation of spurious distortion signals. IMD is most commonly found in active circuits of a radio system, but it can also be found in passive wireless components such as lters, transmission lines, connectors, antennas, attenuators, and so on, especially when transmit power is quite high. Passive intermodulation (PIM) distortion is the name given to the IMD in the latter scenario. With the evolution of radio systems and the scarcity of radio spectrum, PIM interference is being recognized as a potential stumbling block to a radio network's maximum capacity. This article classifies the PIM sources in BS radio systems into two categories, internal and external sources. Internal sources are the radio's passive components such lters, transmission lines, connections, antennas, and so on. External sources, on the other hand, are passive items that are located outside of the BS antenna but inside the RF signal path, such as metallic and rusted objects in the antenna near eld. The high power current flowing through such passive devices can cause nonlinear behavior, resulting in IMD for both types of sources. Also, a review of PIM mitigation techniques is presented in the article.