This paper aims to advance the development of participation in urban design from a substantive standpoint. It departs from a prevailing focus on ideals of participation and describing participatory methods and processes. Instead, the paper stresses the need to acknowledge 'the political' nature of public spaces and how this challenges participatory urban design processes. This leads to a substantive exploration of differences, conflicts and power in the planning and design of public spaces, i.e., unearthing the political. The case of a participatory process in a neighbourhgood of Barcelona illustrates the theoretical discussion. This helps bring forward a much-needed critical and reflective, rather than idealistic, theorization and practice of participation in urban design.