There are important limitations to our work and its interpretation that must be recognized. First, the nature of our exercises changed over time on a number of important dimensions, including the scenario, priority objectives, facilitation, exercise designers and facilitators, and attention to beginning an action plan after the hot wash. As a result of this variation, we are unable to provide a numerical tabulation of the numbers of health departments that struggled with each gap or displayed given strengths. Second, because we did not employ a methodology that could conclusively assess change over time, we cannot be certain that the improvements we identified were truly reflective of improvement, and not due to the inclusion of more sophisticated health departments in the latter part of our exercise period. We doubt this is the case, however, given the national emphasis on preparedness and planning and the ways in which health departments participating in later years qualitatively described their improvement. Furthermore, similar observations regarding improvements in public health preparedness during the same time period have recently been reported by others [24, 25] . We also cannot asses the potential influences that external events (e.g., hurricanes, outbreaks) may have had on health departments during the time period of our work, but it is noteworthy that all exercises were concluded before Hurricane Katrina struck. In addition, our exercises were not conducted in a random sample of health departments, and the findings may not be generalizable to all health departments. Finally, as discussed above, the evidence base for determining best practices in the design and conduct of exercises is extremely thin. We share our experience in the hope that it will help others, but do not propose that our recommendations constitute best, proven practices.