Continuous application of high voltages in an underground cable can cause slow deterioration of its insulation system. Electronic charges can be generated within the medium of the material and this can lead to permanent change of its dielectric properties to withstand high electrical stresses. The long-term degradation of underground cable insulation plays an important role in its eventual breakdown event. One of the ways to assess the insulation condition of an underground cable is by measuring its polarization and depolarization currents (PDC). This technique has been widely used in assessing the performance of transformer insulation system, but can also be used to monitor the degradation within a cable to some extent. The paper aims to describe thoroughly the principle mechanism of PDC and how it can be used to monitor the condition of cable insulation. A simulation model has been developed based on theoretical conjectures of PDC in an ideal cable. The simulation results were then compared with PDC experimental measurements using new unused cables. In addition to this, simulated PDC currents for cables that are subjected to various degrees of degradation were also analyzed in order to deduce severity categorization for replacement exercise of in-service cables. By comparing the simulated data with the experimental results, it is hoped that a clearer understanding on the PDC mechanisms within underground cable insulation can be achieved.