This presentation, based upon the final reports issued by U.S. National Institute of Standards and Technology (NIST), describes how on September 11, 2001. After terrorists flew jet-fuel-laden commercial airliners into the buildings, the aircraft impacts and subsequent fires led to the collapse of the World Trade Center buildings in New York City. NIST complemented in-house expertise with private sector technical experts，accumulated copious documents, photographs, and videos of the disaster，established baseline performance of the buildings，performed computer simulations of the behavior of each building， combined the knowledge gained into a probable collapse sequence for each，conducted over a thousand first-person interviews of building occupants and emergency responders，and analyzed the evacuation and emergency response operations in the buildings. The final reports recommend action in the areas of increased structural integrity, enhanced fire endurance of structures, new methods for fire resistant design of structures, enhanced active fire protection, improved building evacuation, improved emergency response, improved procedures and practices, and education and training.