The purpose of this technical committee report is to present information and review data on the operation of high-voltage direct current (HVDC) transmission systems and their effect on underground/underwater or surface metallic structures, such as pipelines, telephone, electric power, and cable television signal transmission (CATV) cables, railways, reinforced concrete structures, etc. It is intended for individuals associated with the pipeline, water, cable, railway, and electrical transmission and distribution industries. Stray direct current (DC) associated with HVDC transmission systems can result from normal operation of monopolar transmission systems or unbalanced currents in bipolar transmission systems. Bipolar transmission systems can also generate significant levels of stray DC when operating in monopolar operation using the earth as a return circuit as a result of equipment faults or during planned maintenance of the system or converters.