There is a need for emergency planners to accurately plan for and accommodate a potentially significant increase in patient volume in response to a disaster. In addition, an equally large political demand exists for leaders in government and the healthcare sector to develop these capabilities in a financially feasible and evidence-based manner. However, it is important to begin with a clear understanding of this concept on a theoretical level to create this capacity. Intuitively, it is easy to understand that surge capacity describes the ability of a healthcare facility or system to expand beyond its regular operations and accommodate a greater number of patients in response to a multiple casualty-producing event. The way a response to this need is implemented will, of course, vary dramatically depending on numerous issues, including the type of event that has transpired, the planning that has occurred before its occurrence, and the resources that are available. Much has been written on strategies for developing and implementing surge capacity. However, despite the frequency with which the term is used in the medical literature and by the lay press, a clear description of surge capacity as a concept is lacking. The following article will provide this foundation. A conceptual framework of surge capacity will be described, and some new nomenclature will be proposed. This is done to provide the reader with a comprehensive yet simplified view of the various elements that make up the concept of surge capacity. This framework will cover the types of events that can cause a surge of patients, the general ways in which healthcare facilities respond to these events, and the categories of people who would make up the population of affected victims.