A relatively new phenomenon, a flash mob is a spontaneous gathering of people who assemble in a predetermined location, perform an action, and then disperse. Participants network through email, bulletin boards, and websites, and generally assemble silently and without comment before vanishing equally mysteriously. The first recorded flash mob appears to have occurred in Manhattan in June, 2003, when a large crowd of flash mobbers consulted staff at Macy's about purchasing a “love rug” for a suburban commune. Since then, flash mobs of varying sizes have appeared in cities all over the world.

There are two primary ways to organize a flash mob. In the first, participants are given a place, time, and action, as is the case with Pillow Fight Club, a type of flash mob in which participants gather to hold a large pillow fight. If attending the flash mob requires some sort of prop, such as pillow, participants are expected to conceal the props until the starting time of the flash mob, to maintain an air of mystery and to avoid alarming law enforcement. Usually, organizers choose harmless and non-threatening props to avoid antagonizing police, and also ask that participants not involve people who are not actively participating in the flash mob, to avoid upsetting innocent bystanders.

In another organizing technique, participants are told that a flash mob is occurring, but are not given details. Instead, they assemble at multiple pre-staging locations where they are given their assignments and then proceed to the rally point. Organizing in this way prevents news of the details from leaking out, and also allows organizers to stage large and complex events.

Numerous improv and street theatre groups organize flash mobs including No Pants Day and the Mp3 Experiment, two famous flash mobs organized by New York City group Improv Everywhere. The general purpose of a flash mob is to create a light-hearted gathering of people to intrigue and mystify bystanders. In the Mp3 Experiment, for example, participants downloaded musical tracks before attending the event, and spontaneously broke into dance, following instructions embedded in the tracks. During No Pants Day, participants ride the New York City subway system without pants.

A flash mob differs from a political gathering in two primary ways. The first is that organizers of a flash mob do not apply for permits to gather. They rely on the Internet to spread information about the event, and participants try to gather and disperse quickly to evade detainment by law enforcement. A flash mob also usually does not have a political motive; the point of a flash mob is to have fun, not necessarily to make a statement. People who are interested in finding a flash mob in their city to participate in should search for “flash mob” and the region in which they live to find a networking site.