This publication is the result of an Agency Coordinated Research Project that reviewed the procedures and data used in the calibration of plane parallel ionisation chambers for high energy electron and photon beams, as outlined in IAEA TRS-381. It includes data comparing the recommendations of TRS-381 with other codes of practice (DIN 6800-2 and TG-39) as well as independent verification of various correction terms and data applied in TRS-381 (e.g., cavity, fluence and recombination correction factors, and stopping power ratios).