Bike messengers are familiar figures in the downtown cores of major cities. Tasked with delivering time-sensitive materials within, at most, a few hours—and sometimes in as little as fifteen minutes—these couriers ride in all types of weather, weave in and out of dense traffic, dodging (or sometimes failing to dodge) taxis and pedestrians alike in order to meet their clients' tight deadlines. Riding through midtown traffic at breakneck speeds is dangerous work, and most riders do it for very little pay and few benefits. As the courier industry has felt the pressures of first fax machines, then e-mails, and finally increased opportunities for electronic filing of legal "paperwork," many of those who remain in the business are devoted to their job. For these couriers, messengering is the foundation for an all-encompassing lifestyle, an essential part of their identity. In Urban Flow, Jeffrey L. Kidder (a sociologist who spent several years working as a bike messenger) introduces readers to this fascinating subculture, exploring its appeal as well as its uncertainties and dangers.