This report reflects the proceedings of a breakout session, "Surge Capacity: Defining Concepts," at the 2006 Academic Emergency Medicine Consensus Conference, "Science of Surge Capacity." Although there are several general descriptions of surge capacity in the literature, there is no universally accepted standard definition specifying the various components. Thus, the objectives of this breakout session were to better delineate the components of surge capacity and to outline the key considerations when planning for surge capacity. Participants were from diverse backgrounds and included academic and community emergency physicians, economists, hospital administrators, and experts in mathematical modeling. Three essential components of surge capacity were identified: staff, stuff, and structure. The focus on enhancing surge capacity during a catastrophic event will be to increase patient-care capacity, rather than on increasing things, such as beds and medical supplies. Although there are similarities between daily surge and disaster surge, during a disaster, the goal shifts from the day-to-day operational focus on optimizing outcomes for the individual patient to optimizing those for a population. Other key considerations in defining surge capacity include psychosocial behavioral issues, convergent volunteerism, the need for special expertise and supplies, development of a standard of care appropriate for a specific situation, and standardization of a universal metric for surge capacity.