In 2001, letters containing anthrax spores were sent through the mail, contaminating several US buildings. Subsequent sampling revealed that the spores were widely and unevenly spread throughout the buildings and could be found even at locations far from where the letters were handled or opened.  The transport of spores was likely to be influenced by tracking and resuspension by walking people, but the magnitude of this type transport is uncertain and the parameters that control it are not understood. Particles on floors are tracked from one area to another on people's shoes and are resuspended from floors and clothing during walking.  When a hazardous aerosol is released in a building, tracking on footwear influences exposures and alters decontamination requirements.  An understanding of particle resuspension is essential for exposure assessment and risk evaluation during and after the release. This presentation will discuss the results of bench-scale and room-scale laboratory experiments that quantify particle tracking and resuspension by walking people.  Model calculations based on the experimental data are used to explore the following questions: How rapidly are particles transported by tracking from a contaminated floor to nearby areas?  Over what distances do particles spread owing to tracking?  How does resuspension influence exposures during a hazardous aerosol release?