The basis for establishing a measuring method and for setting requirements for the resistance of external walls to impacts by hard objects is described. The assessment procedure, which uses a steel ball impact test, recommends the number and sequence of tests necessary either to measure resistance or to check a wall panel against a specified requirement. Criteria for defining test failure are also proposed. The recommendations for hard body impact resistance proposed for inclusion in a BSI Code of Practice on the design of vertical enclosures are described. These can be related to the results of a survey which examined impact damage and to the measured impact resistance of some claddings.