Recent residential and occupational epidemiological studies indicate a statistical association between 50-60 Hz magnetic field exposure and the risk of developing some kinds of tumors. Several experimental researches have been carried out in vitro and in vivo to verify the possibility that some cell functions may be influenced by ELF (Extremely Low Frequencies: 0-300 Hz) electric and magnetic fields. Such researchers are very important to assess if the statistical association indicated by the epidemiological studies is actually due to a cause-effect relationship between ELF electric and magnetic fields and carcinogenesis. In this review we describe the present state of the experimental research, focusing our attention on the effects of ELF fields on the immune system. We also describe some theoretical researches whose aim is to identify possible mechanisms of interaction between ELF fields and biological systems which may provide biological plausibility to the observed effects.