This article is the result of research that focused on street art and graffiti in the city of Lisbon from 2004 to 2007. The empirical arguments presented draw from ethnographic work and from an analysis of inscriptions on urban walls. In my understanding, these visual manifestations can be understood as political and aesthetic devices, fundamental expressive resources in the negotiation of power and agency in the urban environment. They are vernacular creations that may be interpreted as discursive instruments forged in the context of symbolic struggles, characteristic of the 'field of visibility'. Furthermore, I put forward an analytical framework of graffiti and street art as an urban transgressive grammar, while considering the articulation of produced text and the context of production.