In the project Sonic City, we have developed a system thatenables users to create electronic music in real time by walkingthrough and interacting with the urban environment. Weexplore the use of public space and everyday behaviours forcreative purposes, in particular the city as an interface andmobility as an interaction model for electronic music making.A multi-disciplinary design process resulted in theimplementation of a wearable, context-aware prototype. Thesystem produces music by retrieving information aboutcontext and user action and mapping it to real-time processingof urban sounds. Potentials, constraints, and implications ofthis type of music creation are discussed.