A key aspect of preparing to offer appropriate medical care during a mass gathering is anticipating the number and severity of patient contacts, as well as the number of patients who require transport to an outside hospital. Planning for appropriate medical support is not only critical to providing care to attendees of the event but also to minimize the strain on the surrounding region. The inherent variability between events and circumstances that are unknowable prior to the event makes estimations difficult; however, several prediction models have been developed to provide estimates for the number of medical resources that an event is likely to require. This chapter first discusses the typical number of patient presentations, the required level of care, and hospital transports reported for most mass gatherings. It then provides an overview of the most important factors that influence patient volumes. Four of the most commonly used prediction models are presented and the uses and limitations are discussed. Elements of an event that may lead to significant underestimation then are explored separately. Finally, the risk of critical illness occurring at mass gathering events is considered.