The electrode potential of a metal surface in a corrosive environment (e.g., natural water) is a key indicator of the material’s tendency to corrode. This fact has become more important as technologies for measuring the electrode potential have evolved. Electrode potential measurements are essential for assessing the performance of cathodic protection systems. These measurements have also been important in determining whether reinforcing bar in concrete is corroding. In addition, electrode potential measurements are important in the polarization resistance technique of monitoring corrosion rates. They have also been used to evaluate the tendency of alloys to develop pitting and crevice corrosion in chloride-containing environments. ASTM G215, Standard Guide for Electrode Potential Measurement, covers the measurement of electrode potentials, including the care and maintenance of reference electrodes used for measuring electrode potentials. This new standard is discussed along with more details on the errors and problems that can arise with these measurements.