An international collection of 39 essays covering the history and practice of Critical Mass, the monthly gathering of bicyclists, that takes place in over 400 cities around the world. Whether under the name Critical Mass, Bicicritica, Bicicletada, Massa Critica, Ciemmona, or any of many other names, the grassroots movement of bicyclists riding en masse has changed cities and politics around the world, after its beginnings in San Francisco 20 years ago in 1992. Writers from Italy, Brazil, Mexico, Spain, France, England, Hungary, Czech Republic, Argentina, the United States, and more have contributed eloquent and illuminating accounts of how their respective cities have been altered by the experience of monthly mass cycling.