The adoption of mobile devices as the computers of the 21st century marks a shift away from the fixed terminals that dominated the first 50 years of computing. Associated with this shift will be a new emphasis on context-aware computing. This article examines design approaches to context-aware computing and argues that the evolution of this technology will be characterized by an interplay between top-down systems for command and control and bottom-up systems for collective action. This process will lead to the emergence of “contested-aware cities,” in which power struggles are waged in public spaces with the assistance of context-aware systems.