Critical Mass is an event that was started in San Francisco in 1992 for the purpose of drawing attention to the need for more bicyle lanes and streets. The idea was that if enough cyclists got together for a group ride it would become obvious that the city needed to do something to make the streets more bicycle friendly. Since then the movement has spread all over the world to hundreds of cities. The number of participants can be as few as 20 in smaller towns to as many as tens of thousands. Typically in larger ciites there are several hundred to over a thousand bicycle riders.

Critcal Mass events in most cities are held on the last Friday of the month. They are not formally organized by any group or person and anyone can show up and participate. Some people consider them to be political protests while others enjoy getting together with others who like to use the most green and pomote this environmentally clean form of transportation. In some places the route that is followed is determined by the riders at the front while in others it is decided by vote or by anyone that wants to make up and pass out maps.

The name Critical Mass means that enough bicycle riders participate to take up the entire street to the exclusion of cars and other forms of transportation. Sometimes groups of cyclists will engage in "corkiong" where they line up across side streets blocking cars so that the bicycle riders may go through red lights. Of course this is illegal and sometimes results in arrests. Many cyclists do not like this and believe breaking the law hurts the credibility of the whole process. Many motorists find Critical Mass rides very annoying especially if they are in a hurry to get somewhere. In one case a driver actually gunned his car and ran over several bicyclists. Fortuneatly no one was seriously hurt, although, several of the cyclists attacked and did significant damage to the car. Usually there is tolerance on both sides and such extreme encounters do not occur, although, there have been arrests during some events by police trying to break up the congestion and blocking of traffic. It is difficult for the authorities to charge anyone participating in the ride if it is large since no one is in charge.

Some of the cities where large Critical Mass rides take place are San Francisco, Los Angeles, Boston, New York, Chicago, San Jose, Paris, London and Budapest. A list of locations and times can be found on the Critical Mass Wiki.