A building’s air filtration system can serve as a significant defense component during an airborne chemical and/or biological attack from an external release. To assist building owners and designers in risk analysis and management decisions, this article provides a general introduction to biological and chemical agents considered most likely to be used in a terrorist attack. HVAC system safeguards are discussed, followed by a description of air filters suitable for use in the chemical and biological arena. An internal release directly to the indoor environment poses additional challenges and solutions that are beyond the scope of this article.