Magnetic field effects on plating and etching were discussed in terms of the magnetohydrodynamic (MHD) and micro-MHD effects. These effects result from the Lorentz forces induced by the interaction between the electrolytic current and the magnetic field; however, the scales of length are quite different. The MHD effect emerges in macroscopic scales in the electroplating or electroetching, whereas the micro-MHD effect prevails in the scale of the order of .Em. Therefore, the micro-MHD effect is effective in electroless plating or chemical etching since the electrochemical local cells with a.Em scale of length play important roles. All the effects enhance the mass transfer process through the MHD flow and the micro-MHD flow generated by the Lorentz forces. However, when the mass-transfer process is not ratedetermining, they suppress the rate of plating or etching to level the surface irregularities.