The collapse of the World Trade Center (WTC) in New York provided television viewers worldwide with a firsthand view of the structural phenomenon known as progressive collapse. Although this phenomenon is not new (for example, the Ronan Point apartment building in London collapsed some 40 years previously), the dramatic impact raised questions within the structural engineering community about exactly how the collapse happened, what was the governing mechanism, and how greater robustness might be provided to potentially vulnerable structures. The result has been heightened interest at the fundamental level, considerable research, and the prospect of more realistic representations of the key features of the phenomenon in design procedures.