This book discusses cell response to external electromagnetic and magnetic fields. A new ecological factor, artificial electromagnetic fields (EMF), is a cause for public concern. This book suggests that the cellular response to EMF and to magnetic fields is the base of response of the human organism to EMF. Processes of gene activity are especially important in understanding the EMF influence on cells. The historical review of investigations of cell reactions with special attention to the investigations in the Soviet Union is discussed, as is the EMF-induced increase in cell membrane permeability to ions, mutagenesis, and induction of DNA strand breaks, cell nucleus microelectrophoresis, and the state of chromatin in living cells.