Citizens are increasingly using digital technologies to monitor and sense air quality. Citizen sensing, a term that describes these practices, generates multiple forms of citizen data that can be used to support claims made about environmental problems. This chapter documents how citizen-sensing practices to monitor air quality and changing urban environments in Southeast London generate distinct modes of citizen data, as well as specific formations of data citizens and data relations. Citizen-generated data can become one way to attempt to intervene in and reshape processes of urbanization, especially as they contribute to environmental harm and social injustice. This chapter demonstrates how the “right to breathe” and the “right to the city” become articulated through the right to data, which further indicates how rights are not always self evident or easy to uphold. Citizen data and the practices of data citizens become techniques for reinventing rights by working through concrete struggles to evidence harm and to generate more livable urban worlds.