This paper explains the nature of progressive collapse, with examples, and enlarges considerably on commentary C, supplement No. 4 of the 1975 National Building Code of Canada. Attendant philosophical problems related to code coverage and the design for progressive collapse are considered in discussions of risk, abnormal events that might initiate progressive collapse, and design procedures to reduce the risk of progressive collapse. An example illustrates some of these procedures and indicates how, in a general way, design problems may be approached.