The purpose of this study is to analyze the damage to the structure of each of the WTC Twin Towers due to the high speed impacts of the Boeing 767 airplanes and subsequent fires such as to elucidate why the Twin Towers stood for as long as they did, and why they ultimately collapsed. The Boeing 767 airplane attacks on WTC 1 and WTC 2 caused immediate and significant structural damage to the towers: In each case, exterior columns were severed and the floor system at the point of impact was damaged. The airplanes broke up during the impact and the resulting projectiles and fragments proceeded to inflict further damage to the core. Much of the impact damage to the exterior walls of the towers was evident. However, damage to the interior was not visible and cannot be quantified on the basis of the physical evidence. Dynamic nonlinear explicit finite element FLEX simulations coupled with independently validated airplane crash models were leveraged to understand and assess the structural states of damage to the tower interiors that could not be observed; this includes the degradation or loss of the load carrying capacity of columns and floor assemblies as well as the stripping of fireproofing from structural members. The impact damage to the structure was substantial but so were the reserve capacity and redundancy of the structure. Iterative analyses of the load redistribution in the impact damaged towers clearly indicate that the that the outer tube structure was very effective in developing Vierendeel action around the severed exterior columns and that the outrigger hat truss provided a substantial redundant load path away from the damaged core columns. Although not specifically designed for this purpose, the hat trusses served to delay the eventual collapse of the towers. These analyses also indicate that the damage to the corner of the core in WTC 2 left it in a state more vulnerable to subsequent thermal loads compared to WTC 1. This eccentric damage, more than the height of the airplane impact, resulted in a shorter time to collapse for WTC 2, considering that the fire environments in both towers were not meaningfully different. Further degradation or loss of the load carrying capacity of columns stripped of fireproofing by direct debris impact and heated by fire is shown to be the cause of each collapse. The examination of smoke flow from each building indicates that there were no floor collapses subsequent to the initial impact throughout the fire [1] and our 1 Chief Technology Officer, Weidlinger Associates Inc., 375 Hudson Street, New York, NY 10014. Phone: 212.367.3000. Email: abboud@wai.com. 2 Chairman, Weidlinger Associates Inc., 375 Hudson Street, New York, NY 10014. Phone: 212.367.3000. Email: levy@wai.com. 3 Weidlinger Associates, Inc., 4410 El Camino Real, Los Altos, CA 94022. Phone: 650.949.3010 4 Weidlinger Associates, Inc, 2525 Michigan Avenue, Santa Monica, CA 90404. Phone: 310.998.9154