Public health security-achieved by effectively preventing, detecting, and responding to events that affect public health such as bioterrorism, disasters, and naturally occurring disease outbreaks-is a key aspect of national security. However, effective public health preparedness depends on answering largely unanswerable questions. For example: What is the chance of a bioterror attack in the United States in the next five years? What is the chance of an anthrax attack? What might be the location and magnitude of such an attack? This paper describes how OR-based analyses can provide insight into complex public health preparedness planning problems-and thus support good decisions. Three examples from the author's research are presented: logistics of response to an anthrax attack, prepositioning of medical countermeasures for anthrax, and stockpiling decisions for the United States' Strategic National Stockpile.