On the morning of September 11, 2001, the World Trade Center in New York was attacked by two hijacked airliners. The North Tower, or WTC1 was impacted first, but withstood the collision and subsequent explosion. The South Tower, or WTC2 was attacked minutes later, to the dismay of onlookers, and also withstood the event but collapsed sometime later. Only after this, about 102 minutes after the initial impact did the WTC1 fall. The investigation by NIST (National Institute of Standards and Technology) into what happened and how began about a year after the event and lasted until 2005. The report consisted of many volumes relating mainly to structural details as well as to fire protection problems. The purpose of this book is to provide a review related to correctness and relevance of the NIST report, especially with regard to structural analysis. The most spectacular effect of the aircraft entering a building was a fireball and a thunder, i.e. classical explosion effect. This somehow was not recognized and acknowledged by the investigators. Although the fireballs were mentioned, they were spoken of as merely burning of a combustible material, mainly fuel. The major purpose of the NIST investigation that followed was to determine the mechanism of collapse, i.e. what broke first, what followed etc. to have a better idea how the progressive collapse developed. This was not achieved. Instead, the authors of the report named a certain time point as the beginning of collapse and described the rest in generalities. Without outlining the sequence of failures in the collapsing building it was not possible to identify the design improvements helpful to preclude such failures. A very negative effect on the investigation had the destruction of material evidence, which resulted from some relevant authorities rushing to “clean up” the area. When going through volumes of NIST work it is surprising to see how much effort was applied to decide on matters, which were secondary or irrelevant to the main purpose of the investigation. A more detailed summary of our work is provided in Concluding Review, near the end of this volume. Now, some 20 years after the disaster, the damage done by premature destruction of evidence cannot be undone. However, the analytical part can be corrected and completed. Otherwise, there will be no benefit in terms of learning from the tragedy.