The literature of electrochemistry does not appear to contain a simple equation permitting the calculation of the maximum, or limiting, current density at which a metal can be deposited with 100 per cent current efficiency. Such an equation is now developed on the assumption that the maximum rate of deposition is equal to the maximum rate at which the ions containing the metal to be deposited can diffuse up to the cathode. The values calculated in this manner are shown to be in good agreement with those observed, although discrepancies may arise when there is a possibility of hydrogen evolution.