Abstract: The attack on the World Trade Center (WTC) resulted in a new era of awareness on terrorism in the United States and the issues surrounding the potential for acute and/or long-term health outcomes caused by personal exposures to toxicants released during a terrorist event or an accident. The aftermath of the collapse yielded a situation usually not encountered in environmental health science: a large population's exposure to a previously uncharacterized complex mixture of airborne gases and particles, and re-suspendable particles (>2.5 μm in diameter). This led to a series of rapidly changing potential and actual exposure categories, both in space and time that were associated with the complex mixture of heterogeneous composition and character; e.g., very large particles mixed with much smaller amounts of fine particles, and gases released by uncontrolled combustion. The four categories of outdoor exposure that were encountered will be discussed over the period from September 11 until the fires ended on December 20, 2001. Further, the complex issue of indoor exposure to deposited dust will be highlighted from the beginning through the residual exposure issues being examined today (Category 5 period). The strength of the information on the initial WTC dust and smoke, and the smoke plumes from the fires and the continuing (permanent) gaps in our knowledge within the exposure sciences will be discussed, as well as our attempt to reconstruct exposure for various segments of the population in southern Manhattan and the surrounding areas. This all will be tied to lessons that must be considered in response to future events, natural or otherwise.