Large events have been defined in many ways, from the vague description of a focused gathering of people to the more specific description of an event with at least 1,000 spectators and participants who are gathered at a specific location for a defined period of time. Regardless of the definition applied, the actual medical requirements vary considerably from one event to the next. The ability to predict these medical needs allows for the provision of adequate medical support. Many factors contribute to medical need at a large event, including event type, weather (particularly heat index), the presence of alcohol and / or illicit drugs, the number of participants, event duration, crowd demographics, and venue characteristics. This review will focus on the various features of large events such that the medical planner can better understand the challenge and provide adequate resource for patient care.