This thesis is an in-depth research on information concerning High Voltage Direct Current (HVDC) power transmission. The far reaching effects from faults arising in HVDC cable systems are noteworthy and from this perspective a broad literature review is dedicated to insulation monitoring techniques. The analysis is further subcategorised into on-site and off-site solutions and some existing, operational systems are discussed. Furthermore, the experimental aspect to this work uses dielectric relaxation analysis to assess the integrity of Mass Impregnated HVDC cables. This off-site analysis technique investigates insulation samples using broadband dielectric spectroscopy. It is here proposed that the resulting dielectric relaxation characteristics can be compared to a database of pre-existing spectra as a means to determining the condition of the insulation under investigation.