Seismo-electromagnetic (SEM) effects are the electric and magnetic perturbations caused by the natural geophysical changes associated with earthquakes and volcanic eruptions. These changes appear to take place predominantly in the interval between a few hours and a few days before the originating shock, and they could therefore be used as short-term precursors when the phenomena involved are better understood. Traditionally, SEMs have stirred controversy among researchers. Understanding the generation mechanisms for these phenomena is difficult because a variety of very different physical processes are involved: plasma waves, wave propagation, rock physics, and seismology. Moreover, not all earthquakes and volcanic eruptions appear to produce such perturbations before their onset.