Abstract: Homeland security involves many applications of fluidmechanics and offers many opportunities for research and development. Thisreview explores a wide selection of fluids topics in counterterrorism andsuggests future directions. Broad topics range from preparedness anddeterrence of impending terrorist attacks to detection, response, andrecovery. Specific topics include aircraft hardening, blast mitigation,sensors and sampling, explosive detection, microfluidics and labs-on-a-chip,chemical plume dispersal in urban settings, and building ventilation. Alsodiscussed are vapor plumes and standoff detection, nonlethal weapons,airborne disease spread, personal protective equipment, and decontamination.Involvement in these applications requires fluid dynamicists to think acrossthe traditional boundaries of the field and to work with relateddisciplines, especially chemistry, biology, aerosol science, and atmosphericscience.