The existing underground transmission system in the United States is worth about $5 billion. To optimize the return on that investment and on further investment in the growing system, EPRI has sponsored research to develop periodic and continuous diagnostic procedures and methodologies for evaluating underground cables. By using monitoring results to support maintenance and cable management decisions, utilities will be able to improve maintenance planning, reduce unscheduled outages, increase system reliability, and extend cable life. At the same time, they will be able to operate systems more efficiently and closer to true ratings. The scope of this paper are the following projects: application of dissolved-gas analysis to the periodic monitoring of liquid-filled, paper-wrapped cables (RP7895-1) and also cables insulated with a laminate of cellulose paper, polypropylene film, and cellulose paper, or PPP-insulated cables (RP7910-1); development of a transmission cable diagnostic system based on periodic power factor measurements (RP7910-5); development of an integrated, multifaceted online diagnostic monitoring system known as the Dynamic Rating and Underground Monitoring System (DRUMS) (RP7900); development of a protocol for using perfluorocarbon tracers to locate leaks in high-pressure, liquid-filled cables (RP7905-1); and a long-term cable life evaluation and management project, comprising 10 research areas (RP7914).