Although many authorities define a "mass gathering" as a group exceeding 1,000 persons, several times that number likely are to be present. The event for which the group will gather may be anything from a rock concert to an Olympic competition. Preparations for the event can be minor or major. This article reviews the issues that a physician should consider if he or she chooses to become involved in the delivery of medical care to such populations, as well as the evidence suggesting that a physician should be involved in most such gatherings. Emergency medical care at public gatherings is haphazard at best and dangerous at worst. There are surprisingly few data from which to plan the emergency medical needs for public events and no recognized standards or guidelines for providing emergency medical services at mass public gatherings.