Since several epidemiological studies have indicated an elevated risk for certain types of cancer in both living and working environments where exposure to an extremely low-frequency electromagnetic field (ELF) occurs, public concern about ELF has been increasing because it is impossible to imagine life today without electricity. We reviewed studies on biological effects of ELF with respect to their cytological and biochemical effects, including mutagenicity, clastogenicity and carcinogenicity. The studies can be summarized as follow: 1) There is evidence that outer surface of the cell membrane is the primary locus for ELF-induced cellular alterations. 2) ELF modulate the proliferation of normal as well as transformed cells in vivo and in vitro. The magnitude of the proliferative effects depends on ELF intensity, exposure duration and other cellular factors. 3) No studies clearly demonstrate deleterious effects of ELF exposure on mammalian reproduction and development, but several suggest such effects. 4) Reported evidence does not demonstrate that the ELF acts as a cancer initiator. However, it might act as a promoter or affect tumor progression. Further observations and epidemiological studies of ELF must be accompanied by laboratory experiments to evaluate biological and health effects.