Crosslinked Polyethylene finds application in the insulation of high voltage cables, constituting a high demanding domain in terms of electrical performances. High quality XLPE grades are now produced for insulation under high voltage direct current (HVDC) stress. Luminescence techniques have constituted original techniques along the development of such materials, particularly as regards the role of defects and residues in the behavior of materials in terms of electrical charges stabilization. Luminescence provides a family of extremely sensitive techniques, though limited to substances with unsaturated groups: in materials like XLPE only additives, residues and defects are optically active. After recalling the grounds of luminescence principles in organic materials, we explain the techniques implemented, mainly based on photoluminescence and on an analysis of optical emissions related to charge traps into materials. The main results obtained with the identification of the role of these 'defects' are presented before addressing the knowledge brought by luminescence methods, including electroluminescence, on thermal and electrical ageing aspects of polyethylene materials.