In recent decades, a wide range of abrupt and unpredictable events—including bioterrorism, natural disasters, industrial disasters, and infectious disease outbreaks—have endangered population health and challenged even the most advanced public health and health care systems. Affected populations may face imminent risk to life or health; scarce medical resources; lack of food, water, and shelter; electrical power outages; and interrupted communication. The urgent need to mitigate harms to population health may necessitate rapid decision-making, with incomplete or imperfect information, against a backdrop of political, economic, and social instability. These defining characteristics of public health emergencies are morally significant, not only because they present normatively complex questions about what constitutes an ethical emergency response, but also because they highlight the vital importance of incorporating ethics into public health emergency preparedness.