Only recently have researchers focused on the integration of computing, sensing, and actuation technologies in everyday urban settings and lifestyles. Pervasive computing has largely been applied either in relatively homogeneous unpopulated areas--for example, sensor networks in forests--or in small-scale well-defined built environments, such as smart houses. Urban settings pose many research and deployment challenges: ownership issues are complex as are the dynamics and density of system participation. Despite the complexities, urban computing is, in a limited sense, already a mass phenomenon. Roughly half the world's population lives in urban environments, and connectivity is extensive through short- and long-range communication networks. This special issue looks at urban uses of familiar mobile technologies but focuses mainly on how to produce fully integrated designs specifically for urban settings and how to overcome the deployment challenges.