Most experts define a mass casualty event (MCE) as a natural (e.g., earthquake, pandemic) or manmade (e.g., detonation of a nuclear device, conventional explosive, bioterror attack) incident that suddenly or progressively generates large numbers of injured and/or ill people who require medical and/or mental health care. The magnitude of demand for medical care resources has the potential to vastly outstrip the ability of a health care facility or a local, regional, or national public health and health care delivery system to deliver medical care services consistent with generally established standards of care. The scope and complexity of an MCE can severely challenge even the most highly experienced and well-equipped health care providers and systems. By definition, an MCE generates a level of demand for health care resources that outstrips available supply. Under those circumstances, local and regional health care providers are unable to meet victims' needs at the level normally expected of a modern health care delivery system. Because such situations are difficult to predict and can occur with little or no warning, health care systems and providers must be prepared to swiftly implement contingency plans to reduce less-urgent demand for health care services; optimize the use of existing resources; and secure additional resources, if possible, from backup sources. If these measures are insufficient to meet demand, providers may be forced to shift from the traditional treatment approach, which strives to deliver optimum care to every patient, to one that seeks to do the most good for the most people with the available resources. This latter concept has come to be known as "crisis standards of care." The Institute of Medicine (IOM) Committee on Guidance for Establishing Standards of Care for Use in Disaster Situations published a landmark Letter Report recommending that health care providers, organizations, government officials, and the public approach the challenge in a thoughtful and proactive way, anchored in four values: fairness; equitable processes; community and provider engagement, education, and communication; and the rule of law. The IOM Letter Report also recommended that State plans incorporate, among other things, evidence-based clinical processes and operations. To help Federal, State, and local policymakers, providers, and interested members of the public address the issue with the best available evidence, we were asked to build on the work of the IOM and previous reviews by conducting a thorough review of the evidence regarding allocation of scarce medical resources during MCEs. This report addresses the following Key Questions: Key Question 1. What current or proposed strategies are available to policymakers to optimize the allocation and management of scarce resources during MCEs? What outcomes are associated with these strategies? What factors act as facilitators or barriers to their implementation or effectiveness? Key Question 2. What current or proposed strategies are available to providers to optimize the allocation of scarce resources during MCEs? What outcomes are associated with these strategies? What factors are identified as facilitators or barriers to their implementation or effectiveness? Key Question 3. What are the public's key perceptions and concerns regarding the development and implementation of strategies to allocate and manage scarce resources during actual and potential MCEs? Key Question 4. What current or proposed methods are available to engage providers in discussions regarding the development and implementation of strategies to allocate and manage scarce resources, both in planning for and during an MCE? What outcomes are associated with these strategies? What factors are identified as facilitators or barriers to engaging providers in these discussions?