The US Emergency Use Authorization (EUA) is a critical new tool for medical and public health communities and is applicable for both civilian and military use. It fills the need for timely and practical medical treatment under emergency conditions and authorizes use of the best product available for treatment or prevention when the relevant product has not already been approved or approved for this specific use by the US Food and Drug Administration. The need for and genesis of the EUA, its requirements, its broad application to civilian and military populations, and its features of particular importance to physicians and public health officials are detailed.