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This chapter discusses space-charge storage in HVDC extruded insulation, the relevant manifold effects, and the techniques developed by researchers in order to measure the amount and distribution of space charge stored in the insulation, and the electric field profile associated with such distribution. It reviews the theoretical background of charge injection and transport in insulating polymers. The chapter illustrates the processes that result in space-charge accumulation namely charge generation - via electronic charge injection, field-assisted thermal ionization of impurities, spatially inhomogeneous electric polarization, and steady DC current coupled with a spatially varying ratio ofpermittivity to conductivity. It also reviews the space-charge measurement methods for HVDC extruded insulation. The chapter focuses on the up-to-date developments of the best-suited techniques for measuring space charges in extruded insulation for HVDC cables. It provides a comparison between the best space-charge measurement methods for power cables: pulsed electro acoustic (PEA) versus thermal step method (TSM).