The results of a four-year research and development program specifically directed to the investigation and evaluation of the voltage-dependent phenomena, characteristics, and requirements of dc overhead lines in the +-400 to +-600 kV range are presented. From its inception, the Project was considered investigatory in nature, rather than theoretical, with primary emphasis on the statistical and empirical evaluation of typical line and insulation systems. Although reference is made to the works of other laboratories and investigators and to the (BPA)hvdc Development Program which immediately preceded the Project, the dominant data source is the EPRI-hvdc Dalles Project. Information is included on the ehv dc test facilities at Dalles, Oregon; the corona loss, radio interference, television interference, audible noise, and direct current field effects aspects of overhead ehv dc transmission lines; line and station insulators; clearance requirements; and miscellaneous observations. (LCL)