In this article, Ron explains the important factors a forensic engineer must consider when trying to answer this question – the million dollar question of “why”? Unfortunately, because there is very little that was shared with the public, not enough evidence is present to provide a conclusive answer. In all likelihood, however, given the age of the overpass and the environment it was in, a strong candidate for primary or contributing cause of the collapse is corrosion of the reinforcing steel contained within the concrete. The article discusses this failure mechanism. The article also discusses ways of dealing with rusting of reinforced steel. Most commonly, reinforced concrete members are repaired by removing all loose concrete, sandblasting the steel and applying fresh concrete patches. A proactive maintenance program, which includes scheduled inspections and repairs when required, can reduce the potential for disaster.