In mobile radio communications, often several cellular networks of different operators have to co-exist in the same geographic area. Although the networks of the operators nominally use different frequency bands, interference can occur due to intermodulation, blocking and oscillator noise. The impact of such interferences heavily depends on the system parameters and the scenario, in which the networks are deployed. The interference is most severe, when base stations of two different operators are dislocated. Then, a mobile station MSB far from its serving base station BSB using very high transmit power can be very close to a base station BSA of the other operator A, thus creating very high interference. Similar co-existence situations can occur between different systems which are adjacent in frequency, such as the UMTS TDD and FDD modes in Europe. The main objectives of the research project are to categorize the different interference types, choose suitable measures to evaluate the interactions between the networks, identify critical constellations and parameters, and develop methods to combat interference between different networks for critical cases.