This work studies the chemical composition, physical make-up, and aging mechanisms of thermally-exposed and gamma-irradiated, commercial, cross-linked, polyethylene (XLPE) cable insulation material used in Nuclear Power Plants (NPPs), using various analytical characterization techniques. Understanding of cable aging behaviors is important for cable safety inspection, which is required as both routine inspection and in safety inspection in the process of extending the operating license of NPPs. The fillers and additives in the XLPE cable insulation materials were identified and quantified using scanning electron microscopy, energy dispersive X-ray spectroscopy, carbon-hydrogen elemental weight ratio tests, thermogravimetric analysis, and pyrolysis gas chromatography-mass spectroscopy. Laboratory-based accelerated aging experiments were designed and conducted for aging electrical cables, achieving 240 different aging conditions including thermal-only aging and simultaneous thermal and gamma radiation aging. Aging mechanisms of the XLPE cable insulation materials were studied, including discussion of the changes in antioxidant, flame retardants, and the XLPE polymer chains as the samples age. Additional analytical techniques used for studying the aging mechanisms included gel-fraction measurements, nuclear magnetic resonance spectroscopy, differential scanning calorimetry, permittivity and dielectric loss tangent measurements, and oxidation induction time measurements. A nondestructive aging indicator was identified, that is promising for future development of a nondestructive cable aging testing method.