The core principle used by mobile communication systems to transfer information via the wireless links involves the emission and reception of the electromagnetic field in the radio frequency band. The entire population today is exposed to the electromagnetic field emitted by mobile terminals (mobile phone handsets, cordless phones, tablets, laptops, routers, etc.) and their base stations. The same frequency bands are also used by other technologies, such as radio and television broadcasting, wireless internet access, microwave heating, anti-theft systems, radio frequency identification systems, and so on. When discussing health hazards may be caused by electromagnetic field exposure from mobile communication systems, it must be remembered that in some cases, other sources of electromagnetic exposure may deliver a dominant or a significant exposure component of the total radio frequency electromagnetic exposure of particular individuals. This paper describes the technical aspects of various mobile communication systems. Exposure scenarios with various dominant sources of exposures are discussed in the chapter together with exposure evaluation techniques which may help recognise the exposure pattern.