Uniform measurement procedures and techniques are provided for determining the effectiveness of room-sized, high-performance electromagnetic shielding enclosures at frequencies from 14 kHz to 18 GHz (extendable to 50 Hz and 100 GHz, respectively). The types of enclosures covered include single-shield or double-shield structures of various constructions such as bolted demountable, welded, or integral with building, and made of materials such as steel plate, copper or aluminum sheet, screening hardware cloth, or metal foil. The intent is to reflect current practice and to provide a common reference for suppliers and users on the performance of shielding enclosures.